Electron injector and free electron laser

ABSTRACT

A photocathode comprises a substrate in which a cavity is formed and a film of material disposed on the substrate. The film of material comprises an electron emitting surface configured to emit electrons when illuminated by a beam of radiation. The electron emitting surface is on an opposite side of the film of material from the cavity.

This application incorporates by reference in their entiretiesInternational Application No. PCT/EP2014/075784, filed Nov. 27, 2014,U.S. patent application Ser. No. 15/035,674, fled May 10, 2016, EPPatent Application No. 13195806.8, filed Dec. 5, 2013 and EP PatentApplication No. 14156258.7, filed Feb. 21, 2014 and EP Application No.13196853.9, filed Dec. 12, 2013.

FIELD

The present invention relates to an electron injector for providing anelectron beam to a free electron laser. The electron injector may formpart of an injector arrangement. The free electron laser may be used togenerate radiation for a lithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may for example project a pattern from a patterning device(e.g. a mask) onto a layer of radiation-sensitive material (resist)provided on a substrate.

The wavelength of radiation used by a lithographic apparatus to projecta pattern onto a substrate determines the minimum size of features whichcan be formed on that substrate. A lithographic apparatus which uses EUVradiation, being electromagnetic radiation having a wavelength withinthe range 5-20 nm, may be used to form smaller features on a substratethan a conventional lithographic apparatus (which may for example useelectromagnetic radiation with a wavelength of 193 nm).

EUV radiation for use in one or more lithographic apparatus may beproduced by a free electron laser. A free electron laser may comprise atleast one electron injector arrangement which provides an electron beam,and may comprise at least one injector arrangement.

It is an object of the invention to obviate or mitigate one or moreproblems associated with the prior art.

SUMMARY

According to a first aspect of the invention there is an injectorarrangement for providing an electron beam, comprising a first injectorfor providing a first electron beam, and a second injector for providinga second electron beam, wherein the injector arrangement is operable ina first mode in which the electron beam output from the injectorarrangement is provided by the first injector only and a second mode inwhich the electron beam output from the injector arrangement is providedby the second injector only, wherein the injector arrangement furthercomprises a merging unit configured to merge a recirculating electronbeam with the electron beam provided by the first injector or with theelectron beam provided by the second injector.

In this way the injector can provide the electron beam even when one ofthe first and second injectors is non-operational. This ensures thatfailure of an injector need not cause downtime of an apparatus whichreceives the electron beam, such as a free electron laser. Further, theinjector arrangement of the first aspect allows for maintenance to becarried out on either of the first and second injectors while retainingoperation of the injector arrangement as a whole.

The second injector may be operable to produce an electron beam in thefirst mode. The first injector may be operable to produce an electronbeam in the second mode.

The second injector may be operable to produce the electron beam withelectron bunches at a lower repetition rate in the first mode. The firstinjector may be operable to produce the electron beam with electronbunches at a lower repetition rate in the second mode. For example, theinjector that is not providing an electron beam to the free electronlaser may produce electron bunches having the same charge as theinjector that is supplying the electron beam, but at a very lowrepetition rate (or duty cycle).

The injector arrangement may comprise at least one steering unit. The atleast one steering unit may have a first steering mode in which theelectron beam from the first injector propagates along a first path anda second steering mode in which the electron beam from the firstinjector propagates along a second path.

The at least one steering unit may comprise a third steering mode inwhich the electron beam from the second injector propagates along athird path and a fourth steering mode in which the electron beam fromthe second injector propagates along a fourth path.

The second and fourth paths may, for example, join a path followed bythe electron beam, while the first and third paths may not join the pathfollowed by the electron beam. In this way, when the at least onesteering unit is operating in both the second steering mode and thethird steering mode, the injector arrangement is operating in the firstmode. Conversely, when the at least one steering unit is operating inboth the first mode and the fourth mode, the injector arrangement isoperating in the second mode.

The first injector may be arranged to emit the electron beam along thefirst path and the at least one steering unit may be arranged to divertthe electron beam output by the first injector to propagate along thesecond path when operating in the second steering mode. In this way,active steering of the electron beam emitted by the first injector maynot be required when operating in the first mode.

The second injector may be arranged to emit the electron beam along thethird path and the at least one steering unit may be arranged to divertthe electron beam output by the second injector to propagate along thefourth path when operating in the fourth steering mode. In this way,active steering of the electron beam emitted by the second injector maynot be required when operating in the third mode.

The at least one steering unit may comprise a first steering unitarranged to steer the electron beam from said first injector and asecond steering unit arranged to steer the electron beam from saidsecond injector. The first steering unit may be independently switchablebetween the first steering mode and the second steering mode, and thesecond steering unit may be independently switchable between the thirdsteering mode and the fourth steering mode.

The injector arrangement may further comprise at least one beam dump,wherein the first path leads to said at least one beam dump. The thirdpath may also lead to said at least one beam dump. In this way, eitheror both of the first injectors may be arranged to emit electron beams inthe direction of the beam dump such that the electron beams emitted byeach injector do not need to be actively steered to the beam dump. Thisarrangement may be particularly efficient as a beam dump may be able toreceive electrons from multiple directions (e.g. from both the first andthird paths). Conversely, a target of the electron beam may be able toreceive the electron beam from a single direction.

The injector arrangement may be arranged to direct the electron beamtowards a linear accelerator of a free electron laser. A third steeringunit may be disposed between the injector arrangement and the linearaccelerator, the third steering unit being arranged to steer theelectron beam towards the linear accelerator.

The third steering unit may be a merging unit arranged to merge theelectron beam provided by the injector arrangement with an electron beampropagating in a free electron laser. For example, where the injectorarrangement is used with an ERL FEL, the merging unit may merge theelectron beam provided by the injector arrangement with an electron beamthat has already propagated through the linear accelerator.

The injector arrangement may comprise at least one focusing unitdisposed along a path travelled by the electron beam output by the firstinjector and/or a path of the electron beam output by the secondinjector. In this way, variations in charge distributions withinelectron bunches of the electron beams provided by the first and secondinjectors may be mitigated.

The first injector may be positioned such that a path length of electronbeams between the first injector and a target of the electron beam isgreater than a path length of electron beams between the second injectorand the target; and wherein the at least one focusing unit is arrangedto decrease a size of electron bunches of the electron beam output bythe first injector.

The first injector may be arranged such that the at least one steeringunit bends the electron beam output by the first injector through anangle of less than 90 degrees when operating in the second steeringmode.

The second injector may be arranged such that the at least one steeringunit bends the electron beam output by the second injector through anangle of less than 90 degrees when operating in the fourth steeringmode.

The injector arrangement may be further operable in a third mode inwhich the injector arrangement outputs an electron beam comprisingelectron bunches of the electron beam from the first injectorinterleaved with electron bunches of the electron beam from the secondinjector. In this way, each injector may operate at a repetition ratethat is, for example, half the repetition rate needed for one injectorto provide the electron beam alone. In this way, less wear isexperienced on each injector.

The first injector may be provided in a first room and the secondinjector may be provided in a second room which is shielded from thefirst room.

The injector arrangement may comprise radiation shielding between thefirst injector and the second injector. In this way, it is safe formaintenance or other work to be carried out on one (non-operational)injector while the other injector provides the electron beam.

The first injector may comprise a first photocathode and the secondinjector may comprise a second photocathode. The injector arrangementmay comprise a single photocathode drive laser arranged to provide laserradiation to both the first and second photocathodes.

According to second aspect, there is provided a free electron laserarranged to produce at least one radiation beam comprising the injectorarrangement of the first aspect.

The free electron laser may comprise a merging unit, wherein theinjector arrangement is arranged to provide the electron beam to themerging unit.

According to a third aspect, there is provided a lithographic systemcomprising: a free electron laser according to the second aspectarranged to produce at least one radiation beam; and at least onelithographic apparatus, each of the at least one lithographic apparatusbeing arranged to receive at least one of the at least one radiationbeams. The at least one radiation beam may comprise EUV radiation

The at least one lithographic apparatus may comprise one or more maskinspection apparatus.

According to a fourth aspect of the invention there is provided a freeelectron laser comprising an injector arrangement, comprising a firstelectron beam injector and a second electron beam injector eachconfigured to generate an injected electron beam, and an injector linearaccelerator configured to accelerate the injected electron beam, theinjector linear accelerator being an energy recovery linear accelerator,the free electron laser further comprising a second linear acceleratorand an undulator.

Providing the injector linear accelerator as an energy recovery linearaccelerator is advantageous because it allows energy to be provided tothe injected electron beam which accelerates the electrons and makesthem less susceptible to space charge effects, whilst requiring only alimited amount of energy to be used (compared with using a linearaccelerator which does not use energy recovery). Reducing space chargeeffects allows higher quality electron bunches to be delivered to thesecond linear accelerator.

The injector arrangement may further comprise a merging unit. Themerging unit may be upstream of the injector linear accelerator and maybe configured to switch between operating in a first mode which mergesan injected electron beam from the first injector with a recirculatingelectron beam, and operating in a second mode which merges an injectedelectron beam from the second injector with the recirculating electronbeam.

The merging unit may comprise a combining dipole magnet which isconfigured to bend the recirculating electron beam through a first angleand is configured to bend the injected electron beam through a secondlarger angle. The merging unit may be configured to switch the polarityof the combining dipole magnet when changing between the first andsecond operating modes.

The first injector may be provided on a first side of an axis of theinjector arrangement and the second injector may be provided on a secondopposite side of the axis of the injector arrangement.

The merging unit may comprise a plurality of dipole magnets arranged asa chicane which bends the recirculating electron beam, the polarity ofthe dipole magnets being switchable to reverse the direction of bendapplied to the recirculating electron beam.

The chicane may be configured to direct the recirculating beam towardsthe combining dipole magnet from the same side of the injectorarrangement axis as the first injector when the merging unit isoperating in the first mode, and is configured to direct therecirculating beam towards the combining dipole magnet from the sameside of the injector arrangement axis as the second injector when themerging unit is operating in the second mode.

The chicane may be configured to deliver the recirculating beam to thecombining dipole magnet at an angle and with a spatial position selectedsuch that the recirculating electron beam will propagate along theinjector arrangement axis when it leaves the combining dipole magnet.

The chicane dipole magnets may be electromagnets.

The first injector may comprise a plurality of dipole magnets and aplurality of quadrupole magnets configured, when the merging unit isoperating in the first mode, to deliver the first injected electron beamto the combining dipole magnet at an angle and with a spatial positionselected such that the first injected electron beam will propagate alongthe injector arrangement axis when it leaves the combining dipolemagnet.

The second injector may comprise a plurality of dipole magnets and aplurality of quadrupole magnets configured, when the merging unit isoperating in the second mode, to deliver the second injected electronbeam to the combining dipole magnet at an angle and with a spatialposition selected such that the second injected electron beam willpropagate along the injector arrangement axis when it leaves thecombining dipole magnet.

The dipole magnets and quadrupole magnets may be tuned to provideelectron bunches with a desired quality after the injector linearaccelerator.

The first injector and the second injector may both be configured toprovide the injected electron beam with an energy which is below athreshold energy at which electrons would induce radioactivity in a beamdump.

The injector linear accelerator may be configured to increase the energyof the injected electron beam by at least 20 MeV.

The first injector may be provided in a first room and the secondinjector may be provided in a second room, each room having walls whichprovide shielding from electromagnetic radiation.

The second linear accelerator may be an energy recovery linearaccelerator.

The second linear accelerator may be configured to increase the energyof the electron beam by 100 MeV or more after acceleration by theinjector linear accelerator.

The free electron laser may comprise a first loop containing the secondlinear accelerator and the injector linear accelerator and a second loopcontaining the second linear accelerator and the undulator. The pathlength of the first loop may be equal to the path length of the secondloop.

The first injector and the second injector may each be configured toprovide electron beam clearance gaps at a rate which corresponds withthe time required for electrons to travel around the first loop.

The first injector and the second injector may be provided in roomswhich are above the second linear accelerator and the undulator.

According to a fifth aspect of the invention there is provided a methodof producing a radiation beam using a free electron laser, the methodcomprising using a first electron beam injector to generate an injectedelectron beam and combining the injected electron beam with arecirculating electron beam, or using a second electron beam injector togenerate an injected electron beam and combining the injected electronbeam with the recirculating electron beam, using an injector linearaccelerator to increase the energy of the injected electron beam bytransferring energy from the recirculating electron beam to the injectedelectron beam, using a second linear accelerator to further increase theenergy of the injected electron beam, and using an undulator to generatethe radiation beam using the electron beam.

According to a sixth aspect of the invention there is provided anelectron injector comprising a support structure arranged to support aphotocathode, a beam delivery system arranged to direct a beam ofradiation from a radiation source onto a region of the photocathodethereby causing the photocathode to emit a beam of electrons, anadjustment mechanism operable to change the region of the photocathodewhich is illuminated by the radiation beam; and a steering unit operableto apply a force to the beam of electrons to alter its trajectory suchthat the electrons become substantially coincident with an axis of theelectron injector.

The region of the photocathode which is illuminated by the radiationbeam may become damaged during use. For instance over time the peakcurrent of the electron beam emitted from the region may decrease and/orthe emittance of the electron beam may increase. Changing the region ofthe photocathode which is illuminated by the radiation beam allows theregion of the photocathode from which the electron beam is emitted to bemoved around the photocathode. This extends the useful lifetime of thephotocathode by utilising a larger extent of the photocathode to emitelectrons over the lifetime of the photocathode.

Changing the region of the photocathode which is illuminated by theradiation beam may displace the beam of electrons emitted from thephotocathode from the axis of the electron injector. The axis mayrepresent the desired trajectory of the electron beam upon leaving theelectron source. The steering unit corrects for the displacement fromthe axis and thus causes the electrons to be coincident with the axisupon leaving the electron injector. This may also have the effect ofseparating the path of the electrons in the electron injector from thepath of ions which are created downstream of the electron injector. Theions may collide with the photocathode and cause damage to thephotocathode. By separating the path of the ions from the path of theelectrons in the electron injector, the region of the photocathode whichsuffers damage from ion collisions may be separated from the region ofthe photocathode from which the beam of electrons is emitted. Thisfurther extends the useful lifetime of the photocathode.

The steering unit may comprise one or more electromagnets.

The steering unit may be downstream of an electron booster of theelectron injector.

The region of the photocathode which is illuminated by the radiationbeam may be separated from the axis of the electron injector.

The beam delivery system may be configured such that the radiation beamis not perpendicular to the photocathode when it is incident upon thephotocathode.

The adjustment mechanism may comprise a radiation beam adjustment unitin the beam delivery system, the radiation beam adjustment unit beingoperable to change one or more properties of the radiation beam.

The radiation beam adjustment unit may be operable to change thedirection of propagation of the radiation beam.

The beam delivery system may comprise a mirror arranged to reflect theradiation beam onto a region of the photocathode and the adjustmentmechanism may comprise an actuator operable to change the positionand/or the orientation of the mirror.

The adjustment mechanism may be operable to control the shape of theregion of the photocathode which is illuminated by the radiation beam.

The adjustment mechanism may be operable to control the shape of theregion of the photocathode which is illuminated by the radiation beamsuch that the beam of electrons emitted from the illuminated regiontakes on one or more desired properties after the steering unit appliesa force to the beam of electrons.

The adjustment mechanism may comprise an actuator operable to change theposition and/or the orientation of the photocathode.

The actuator may be operable to rotate the photocathode.

The electron injector may further comprise a controller wherein thecontroller is operable to control the adjustment mechanism in order tocontrol the change of the region of the photocathode which isilluminated by the radiation beam.

The steering unit may be operable to adjust the force which is appliedto the beam of electrons in response to the region of the photocathodewhich is illuminated by the radiation beam.

The controller may control the adjustment of the force which is appliedto the beam of electrons in response to the change of the region of thephotocathode which is illuminated by the radiation beam.

The electron injector may further comprise an electron beam measurementdevice operable to measure one or more properties of the beam ofelectrons.

The steering unit may be operable to adjust the force which is appliedto the beam of electrons in response to the measurement of one or moreproperties of the beam of electrons.

The radiation source may be a laser and the beam of radiation may be alaser beam.

The laser may be a picosecond laser.

The beam of electrons may comprise a plurality of bunches of electrons.

The electron injector may further comprise an electron booster operableto accelerate the beam of electrons.

The axis may represent a desired trajectory of the beam of electronswhich is output from the electron injector.

The support structure may be housed in an electron gun and the electroninjector may further comprise an actuator operable to adjust theposition and/or the orientation of the electron gun.

The actuator may be operable to adjust the position and/or theorientation of the electron gun in response to the change in the regionof the photocathode which is illuminated by the radiation beam.

According to a seventh aspect of the invention there is provided a freeelectron laser comprising the electron injector according to the sixthaspect, a particle accelerator operable to accelerate the beam ofelectrons to relativistic speeds and an undulator operable to cause therelativistic electrons to follow an oscillating path thereby causingthem to stimulate emission of coherent radiation.

The undulator may be configured to cause the relativistic electrons toemit EUV radiation.

The particle accelerator may be a linear accelerator.

According to an eighth aspect of the invention there is provided alithographic system comprising a free electron laser according to thesixth aspect of the invention and one or more lithographic apparatus.

According to a ninth aspect of the invention there is provided a methodof producing an electron beam using an electron injector, the methodcomprising directing a beam of radiation onto a region of a photocathodethereby causing the photocathode to emit a beam of electrons, changingthe region of the photocathode which is illuminated by the radiationbeam and applying a force to the beam of electrons to alter itstrajectory such that the electrons are coincident with an axis of theelectron injector.

Applying a force to the beam of electrons may comprise using one or moreelectromagnets to generate a magnetic field so as to alter thetrajectory of the beam of electrons.

The region of the photocathode which is illuminated by the radiationbeam may be separated from the axis of the electron injector.

Changing the region of the photocathode which is illuminated by theradiation beam may comprise changing one or more properties of theradiation beam.

Changing the region of the photocathode which is illuminated by theradiation beam may comprise changing the position and/or the orientationof the photocathode.

Changing the position and/or the orientation of the photocathode maycomprise rotating the photocathode.

The method may further comprise adjusting the force which is applied tothe beam of electrons in response to the change of the region of thephotocathode which is illuminated by the radiation beam.

The method may further comprise measuring one or more properties of thebeam of electrons.

The method may further comprise adjusting the force which is applied tothe beam of electrons in response to the measurement of one or moreproperties of the beam of electrons.

The method may further comprise controlling the shape of the region ofthe photocathode which is illuminated by the radiation beam.

The shape of the region of the photocathode which is illuminated by theradiation beam may be controlled such that the beam of electrons emittedfrom the illuminated region takes on one or more desired propertiesafter applying the force to the beam of electrons.

The beam of electrons may comprise a plurality of bunches of electrons.

The axis may represent a desired trajectory of the beam of electronswhich is output from the electron injector.

According to a tenth aspect there is provided a method for producingradiation comprising producing an electron beam according to the methodof the ninth aspect accelerating the beam of electrons to relativisticspeeds and causing the relativistic electrons to follow an oscillatingpath thereby causing them to stimulate emission of coherent radiation.

The relativistic electrons may be caused to stimulate emission of EUVradiation.

According to an eleventh aspect of the invention there is provided aphotocathode comprising a substrate in which a cavity is formed and afilm of material disposed on the substrate, wherein the film of materialcomprises an electron emitting surface configured to emit electrons whenilluminated by a beam of radiation, wherein the electron emittingsurface is on an opposite side of the film of material from the cavity.

The photocathode may be subjected to collisions with ions. The cavity inthe substrate serves to increase the depth into the substrate at whichthe ions are stopped in the photocathode. This decreases an amount ofmaterial which is sputtered from photocathode due to ion collisions.Sputtered material may be deposited onto the photocathode and may reducethe photocathode's quantum efficiency. By reducing the amount ofmaterial which is sputtered from the photocathode, any reduction in thequantum efficiency may be reduced. This increases the useful lifetime ofthe photocathode.

Reducing the amount of material which is sputtered from the photocathodemay also prevent gradients of quantum efficiency from developing on thephotocathode. This increases the stability of the current associatedwith an electron beam emitted from the photocathode and increases theuniformity of the charge distribution of electron bunches emitted fromthe photocathode. Both of these effects are particularly advantageouswhen using the photocathode in an electron source for a free electronlaser.

Increasing the depth into the substrate at which ions are stopped in thephotocathode also acts to decrease the heating of the photocathode closeto the surface of the photocathode. This reduces any thermionic emissionof electrons from the photocathode. This reduces a dark current emittedby the photocathode. This is advantageous when using the photocathode inan electron source for a free electron laser since it reduces strayelectrons in the free electron laser which may damage components of thefree electron laser.

The photocathode may include an impact region which will receive ionsduring operation of the photocathode.

The cavity in the substrate may be substantially aligned with the impactregion.

The thickness of a portion of the photocathode disposed between theelectron emitting surface and the cavity may be sufficiently thin thatpositively charged ions incident at that portion of the photocathodepass through that portion of the photocathode and into the cavity.

The thickness of the portion of the photocathode disposed between theelectron emitting surface and the cavity may be less than 10 microns.

The photocathode may be configured to take on a desired shape after adeformation of the photocathode which is brought about by anelectrostatic pressure applied to the photocathode when the photocathodeis held at a voltage.

The photocathode may be configured such that after the deformation ofthe photocathode electric field lines associated with the voltageapplied to the photocathode are substantially uniform.

The substrate may comprise an indentation in the substrate.

The cavity in the substrate may comprise a chamfer.

The substrate may comprise one or more ribs.

The one or more ribs may be arranged to strengthen the photocathode toresist an electrostatic pressure applied to the photocathode when thephotocathode is held at a voltage.

The ribs may be arranged in a honeycomb structure.

The ribs may have a thickness of less than approximately 1 micron.

The substrate may comprise silicon.

The film of material may comprise one or more alkali metals.

The film of material may comprise sodium potassium antimonide.

According to a twelfth aspect of the invention there is provided anelectron injector comprising a photocathode according to the eleventhaspect of the invention arranged to receive a beam of radiation from aradiation source, and an electron booster operable to accelerate a beamof electrons emitted from the photocathode.

According to a thirteenth aspect of the invention there is provided afree electron laser comprising an electron source according to thetwelfth aspect of the invention, a linear accelerator operable toaccelerate the beam of electrons to relativistic speeds, and anundulator operable to cause the relativistic electrons to follow anoscillating path thereby causing them to stimulate emission of coherentradiation.

The undulator may be configured to cause the electrons to emit EUVradiation.

According to a fourteenth aspect of the invention there is provided alithographic system comprising a free electron laser according to thethirteenth aspect and one or more lithographic apparatus.

According to a fifteenth aspect of the invention there is provided amethod of producing an electron beam comprising directing a beam ofradiation to be incident on a region of a photocathode thereby causingthe photocathode to emit a beam of electrons, wherein the photocathodecomprises a substrate in which a cavity is formed and a film of materialdisposed on the substrate, wherein the film of material is configured toemit electrons from an electron emitting surface when illuminated by thebeam of radiation, and wherein the electron emitting surface is on anopposite side of the film of material from the cavity.

Features of any aspect of the invention may be combined with features ofany other aspect or aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 is a schematic illustration of a lithographic system comprising afree electron laser according to an embodiment of the invention andeight lithographic apparatuses;

FIG. 2 is a schematic illustration of a lithographic apparatus thatforms part of the lithographic system of FIG. 1;

FIG. 3 is a schematic illustration of a free electron laser according toan embodiment of the invention;

FIG. 4 is a schematic illustration of an injector arrangement accordingto an embodiment of the invention;

FIG. 5 is a schematic illustration of an injector arrangement accordingto an alternative embodiment of the invention;

FIG. 6 is a schematic illustration of an injector arrangement accordingto an alternative embodiment of the invention;

FIG. 7 is a schematic illustration of an injector arrangement accordingto an alternative embodiment of the invention;

FIG. 8 is a schematic illustration of an injector arrangement accordingto an alternative embodiment of the invention;

FIG. 9 is a schematic illustration of a drive mechanism for a pluralityof injectors according to a further embodiment of the invention;

FIG. 10 is a schematic illustration of a free electron laser accordingto an embodiment of the invention;

FIG. 11 is a schematic illustration of an injector arrangement which mayform part of the free electron laser of FIG. 10;

FIG. 12 is a schematic illustration of an electron injector according toan embodiment of the invention;

FIG. 13 is a schematic illustration of a photocathode for use in theelectron injector of FIG. 12;

FIG. 14 is a schematic illustration of a cross-section through aphotocathode which may be used by the electron injector of FIG. 13;

FIG. 15 is a representation of the stopping positions of ions in asilicon substrate;

FIG. 16 is a schematic illustration of a cross-section through aphotocathode subjected to electrostatic pressure;

FIG. 17a is a schematic illustration of a cross-section through aphotocathode configured to take on a desired shape upon exposure to anelectrostatic pressure;

FIG. 17b is a schematic illustration of a cross-section through thephotocathode of FIG. 17a whilst being subjected to an electrostaticpressure;

FIG. 18 is a schematic illustration of a plan view of a portion of asubstrate comprising reinforcing ribs; and

FIG. 19 is a schematic illustration of a cross-section through aphotocathode comprising a cavity with an opening.

DETAILED DESCRIPTION

FIG. 1 shows an example of a lithographic system LS according to oneembodiment of the invention. The lithographic system LS comprises aradiation source in the form of a free electron laser FEL, a beamsplitting apparatus 19 and eight lithographic apparatuses LA1-LA8. Theradiation source is configured to generate an extreme ultraviolet (EUV)radiation beam B (which may be referred to as a main beam). The mainradiation beam B is split into a plurality of radiation beams B_(a)B_(h)(which may be referred to as branch beams), each of which is directed toa different one of the lithographic apparatuses LA1-LA8, by the beamsplitting apparatus 19. The branch radiation beams B_(a)B_(h) may besplit off from the main radiation beam in series, with each branchradiation beam being split off from the main radiation beam downstreamfrom the preceding branch radiation beam. Where this is the case thebranch radiation beams may for example propagate substantially parallelto each other.

The lithographic system LS further comprises beam expanding optics 20.The beam expanding optics 20 are arranged to increase the crosssectional area of the radiation beam B. Advantageously, this decreasesthe heat load on mirrors downstream of the beam expanding optics 20.This may allow the mirrors downstream of the beam expanding optics to beof a lower specification, with less cooling, and therefore lessexpensive. Additionally or alternatively, it may allow the downstreammirrors to be nearer to normal incidence. The beam splitting apparatus19 may comprise a plurality of static extraction mirrors (not shown)arranged in the path of the beam B which direct radiation from the mainbeam B along the plurality of branch radiation beams B_(a)B_(h).Increasing the size of the main beam B reduces the accuracy with whichthe mirrors must be located in the beam B path. Therefore, this allowsfor more accurate splitting of the output beam B by the splittingapparatus 19. For example, the beam expanding optics 20 may be operableto expand the main beam B from approximately 100 μm to more than 10 cmbefore the main beam B is split by the beam splitting apparatus 19.

The radiation source FEL, beam splitting apparatus 19, beam expandingoptics 20 and lithographic apparatuses LA1-LA8 may all be constructedand arranged such that they can be isolated from the externalenvironment. A vacuum may be provided in at least part of the radiationsource FEL, beam expanding optics 20, beam splitting apparatus 19 andlithographic apparatuses LA1-LA8 so as to minimise the absorption of EUVradiation. Different parts of the lithographic system LS may be providedwith vacuums at different pressures (i.e. held at different pressureswhich are below atmospheric pressure).

Referring to FIG. 2, a lithographic apparatus LA1 comprises anillumination system IL, a support structure MT configured to support apatterning device MA (e.g. a mask), a projection system PS and asubstrate table WT configured to support a substrate W. The illuminationsystem IL is configured to condition the branch radiation beam B_(a)that is received by that lithographic apparatus LA1 before it isincident upon the patterning device MA. The projection system PS isconfigured to project the radiation beam B_(a)′ (now patterned by themask MA) onto the substrate W. The substrate W may include previouslyformed patterns. Where this is the case, the lithographic apparatusaligns the patterned radiation beam B_(a)′ with a pattern previouslyformed on the substrate W.

The branch radiation beam B_(a) that is received by the lithographicapparatus LA1 passes into the illumination system IL from the beamsplitting apparatus 19 though an opening 8 in an enclosing structure ofthe illumination system IL. Optionally, the branch radiation beam B_(a)may be focused to form an intermediate focus at or near to the opening8.

The illumination system IL may include a facetted field mirror device 10and a facetted pupil mirror device 11. The faceted field mirror device10 and faceted pupil mirror device 11 together provide the radiationbeam B_(a) with a desired cross-sectional shape and a desired angulardistribution. The radiation beam B_(a) passes from the illuminationsystem IL and is incident upon the patterning device MA held by thesupport structure MT. The patterning device MA reflects and patterns theradiation beam to form a patterned beam B_(a)′. The illumination systemIL may include other mirrors or devices in addition to or instead of thefaceted field mirror device 10 and faceted pupil mirror device 11. Theillumination system IL may for example include an array of independentlymoveable mirrors. The independently moveable mirrors may for examplemeasure less than 1 mm across. The independently moveable mirrors mayfor example be microelectromechanical systems (MEMS) devices.

Following reflection from the patterning device MA the patternedradiation beam B_(a)′ enters the projection system PS. The projectionsystem PS comprises a plurality of mirrors 13, 14 which are configuredto project the radiation beam B_(a)′ onto a substrate W held by thesubstrate table WT. The projection system PS may apply a reductionfactor to the radiation beam, forming an image with features that aresmaller than corresponding features on the patterning device MA. Areduction factor of 4 may for example be applied. Although theprojection system PS has two mirrors 13, 14 in FIG. 2, the projectionsystem may include any number of mirrors (e.g. six mirrors).

As described above, the radiation source configured to generate an EUVradiation beam B comprises a free electron laser FEL. A free electronlaser comprises a source, which is operable to produce a bunchedrelativistic electron beam, and a periodic magnetic field through whichthe bunches of relativistic electrons are directed. The periodicmagnetic field is produced by an undulator and causes the electrons tofollow an oscillating path about a central axis. As a result of theacceleration caused by the magnetic structure the electronsspontaneously radiate electromagnetic radiation generally in thedirection of the central axis. The relativistic electrons interact withradiation within the undulator. Under certain conditions, thisinteraction causes the electrons to bunch together into microbunches,modulated at the wavelength of radiation within the undulator, andcoherent emission of radiation along the central axis is stimulated.

The path followed by the electrons may be sinusoidal and planar, withthe electrons periodically traversing the central axis, or may behelical, with the electrons rotating about the central axis. The type ofoscillating path may affect the polarization of radiation emitted by thefree electron laser. For example, a free electron laser which causes theelectrons to propagate along a helical path may emit ellipticallypolarized radiation, which may be preferred for exposure of a substrateW by some lithographic apparatuses.

FIG. 3 is a schematic depiction of a free electron laser FEL comprisingan injector arrangement 21, a linear accelerator 22, an undulator 24 anda beam dump 100. A free electron laser FEL is also schematicallydepicted in FIG. 10 (as described further below).

The injector arrangement 21 of the free electron laser FEL is arrangedto produce a bunched electron beam E and comprises an electron sourcesuch as, for example, a thermionic cathode or photocathode and anaccelerating electric field. The electron beam E is accelerated torelativistic speeds by the linear accelerator 22. In an example, thelinear accelerator 22 may comprise a plurality of radio frequencycavities, which are axially spaced along a common axis, and one or moreradio frequency power sources, which are operable to control theelectromagnetic fields along the common axis as bunches of electronspass between them so as to accelerate each bunch of electrons. Thecavities may be superconducting radio frequency cavities.Advantageously, this allows: relatively large electromagnetic fields tobe applied at high duty cycles; larger beam apertures, resulting infewer losses due to wakefields; and for the fraction of radio frequencyenergy that is transmitted to the beam (as opposed to dissipated throughthe cavity walls) to be increased. Alternatively, the cavities may beconventionally conducting (i.e. not superconducting), and may be formedfrom, for example, copper. Other types of linear accelerators may alsobe used. For example, the linear accelerator 22 may comprise a laseraccelerator, wherein the electron beam E passes through a focused laserbeam and the electric field of the laser beam causes the electrons toaccelerate.

The injector arrangement 21 and linear accelerator 22 together providerelativistic electrons.

Optionally, the electron beam E may pass through a bunch compressor 23.The bunch compressor 23 may be disposed downstream or upstream of thelinear accelerator 22. The bunch compressor 23 is configured to bunchelectrons in the electron beam E and spatially compress existing bunchesof electrons in the electron beam E. One type of bunch compressor 23comprises a radiation field directed transverse to the electron beam E.An electron in the electron beam E interacts with the radiation andbunches with other electrons nearby. Another type of bunch compressor 23comprises a magnetic chicane, wherein the length of a path followed byan electron as it passes through the chicane is dependent upon itsenergy. This type of bunch compressor may be used to compress a bunch ofelectrons which have been accelerated in a linear accelerator 22 using aplurality of radio frequency cavities.

The electron beam E then passes through the undulator 24. Generally, theundulator 24 comprises a plurality of magnets, which are operable toproduce a periodic magnetic field and arranged so as to guide therelativistic electrons produced by the injector arrangement 21 andlinear accelerator 22 along a periodic path. As a result, the electronsradiate electromagnetic radiation generally in the direction of acentral axis of the undulator 24. The undulator 24 comprises a pluralityof modules, each section comprising a periodic magnet structure. Theelectromagnetic radiation may form bunches (referred to as photonbunches herein) at the beginning of each undulator module. The undulator24 further comprises a mechanism for refocusing the electron beam E suchas, for example, a quadrupole magnet in between one or more pairs ofadjacent modules. The mechanism for refocusing the electron beam E mayreduce the size of the electron bunches, which may improve the couplingbetween the electrons and the radiation within the undulator 24,increasing the stimulation of emission of radiation.

The free electron laser FEL may operate in self-amplified stimulatedemission (SASE) mode. Alternatively, the free electron laser FEL maycomprise a seed radiation source, which may be amplified by stimulatedemission within the undulator 24. A beam of radiation B propagates fromthe undulator 24. The radiation beam B comprises EUV radiation.

As electrons move through the undulator 24, they interact with theelectric field of the radiation, exchanging energy with the radiation.In general the amount of energy exchanged between the electrons and theradiation will oscillate rapidly unless conditions are close to aresonance condition, given by:

$\begin{matrix}{{\lambda_{em} = {\frac{\lambda_{u}}{2\gamma^{2}}( {1 + \frac{K^{2}}{A}} )}},} & (1)\end{matrix}$where λ_(em) is the wavelength of the radiation, λ_(u) is the undulatorperiod, γ is the Lorentz factor of the electrons and K is the undulatorparameter. A is dependent upon the geometry of the undulator 24: for ahelical undulator A=1, whereas for a planar undulator A=2. In practice,each bunch of electrons will have a spread of energies although thisspread may be minimised as far as possible (by producing an electronbeam EB₁ with low emittance). The undulator parameter K is typicallyapproximately 1 and is given by:

$\begin{matrix}{{K = \frac{q\;\lambda_{u}B_{0}}{2\pi\;{mc}}},} & (2)\end{matrix}$where q and m are, respectively, the electric charge and mass of theelectrons, B0 is the amplitude of the periodic magnetic field, and c isthe speed of light.

The resonant wavelength λ_(em) is equal to the first harmonic wavelengthspontaneously radiated by electrons moving through the undulator 24. Thefree electron laser FEL may operate in self-amplified stimulatedemission (SASE) mode. Operation in SASE mode may require a low energyspread of the electron bunches in the electron beam EB₁ before it entersthe undulator 24. Alternatively, the free electron laser FEL maycomprise a seed radiation source, which may be amplified by stimulatedemission within the undulator 24.

Electrons moving through the undulator 24 may cause the amplitude ofradiation to increase, i.e. the free electron laser FEL may have anon-zero gain. Maximum gain may be achieved when the resonance conditionis met or when conditions are close to but slightly off resonance.

An electron which meets the resonance condition as it enters theundulator 24 will lose (or gain) energy as it emits (or absorbs)radiation, so that the resonance condition is no longer satisfied.Therefore, in some embodiments the undulator 24 may be tapered. That is,the amplitude of the periodic magnetic field and/or the undulator periodλ_(u) may vary along the length of the undulator 24 in order to keepbunches of electrons at or close to resonance as they are guided thoughthe undulator 24. Note that the interaction between the electrons andradiation within the undulator 24 produces a spread of energies withinthe electron bunches. The tapering of the undulator 24 may be arrangedto maximise the number of electrons at or close to resonance. Forexample, the electron bunches may have an energy distribution whichpeaks at a peak energy and the tapering maybe arranged to keep electronswith this peak energy at or close to resonance as they are guided thoughthe undulator 24. Advantageously, tapering of the undulator has thecapacity to significantly increase conversion efficiency. The use of atapered undulator may increase the conversion efficiency (i.e. theportion of the energy of the electron beam E which is converted toradiation in the radiation beam B) by more than a factor of 2. Thetapering of the undulator may be achieved by reducing the undulatorparameter K along its length. This may be achieved by matching theundulator period λ_(u) and/or the magnetic field strength B₀ along theaxis of the undulator to the electron bunch energy to ensure that theyare at or close to the resonance condition. Meeting the resonancecondition in this manner increases the bandwidth of the emittedradiation.

After leaving the undulator 24, the electromagnetic radiation (photonbunch) is emitted as the radiation beam B. The radiation beam B suppliesEUV radiation to the lithographic apparatuses LA1 to LA8. The radiationbeam B may optionally be directed to dedicated optical components whichmay be provided to direct the radiation beam B from the free electronlaser FEL to the lithographic apparatuses LA1 to LA8. Since EUVradiation is generally well absorbed by all matter, reflective opticalcomponents are used (rather than transmissive components) so as tominimise losses. The dedicated optical components may adapt theproperties of the radiation beam produced by the free electron laser FELso that it is suitable for acceptance by the illumination systems IL ofthe lithographic apparatuses LA1 to LA8. The dedicated opticalcomponents may include the expander optics 20 and the beam splitter 19depicted in FIG. 1.

After leaving the undulator 24, the electron beam E is absorbed by adump 100, which may, for example, include a large amount of water or amaterial with a high threshold for radioactive isotope generation byhigh energy electron impact, for example Al with threshold ofapproximately 15 MeV. Before passing to the dump 100, it may bedesirable to extract energy from the electron beam E to reduce itsradioactivity and/or to recover at least part of the energy.

In order to reduce the energy of the electrons before they are absorbedby the beam dump 100, an electron decelerator 26 may be disposed betweenthe undulator 24 and the beam dump 100. The electron decelerator 26reduces the amount of energy the electrons have when they are absorbedby the beam dump 100 and will therefore reduce the levels of inducedradiation and secondary particles produced in the beam dump 100. Thisremoves, or at least reduces, the need to remove and dispose ofradioactive waste from the beam dump 100. This is advantageous since theremoval of radioactive waste requires the free electron laser FEL to beshut down periodically and the disposal of radioactive waste can becostly and can have serious environmental implications.

The electron decelerator 26 may be operable to reduce the energy of theelectrons to below a threshold energy. Electrons below this thresholdenergy may not induce any significant level of radioactivity in the beamdump 100. During operation of the free electron laser FEL, gammaradiation will be present but advantageously when the electron beam E isswitched off, the beam dump 100 will be safe to handle.

Components of injectors (such as cathodes) may have a relatively shortoperational lifetime and may require frequent replacement ormaintenance. Such replacement and maintenance has a detrimental effecton the free electron lasers of which they form a part. Embodiments ofthe invention may provide two injectors, the injectors being configuredsuch that one injector may be switched off to allow for maintenancewhilst the other injector is operating. Embodiments of the invention mayincrease the operational lifetime of injector components such ascathodes.

Referring to FIG. 4 an injector arrangement 21 comprises a firstinjector 30 and a second injector 31. Each injector 30, 31 comprises itsown electron source such as, for example, a thermionic cathode orphotocathode, and an accelerating electric field. A first injector 30 isarranged to produce a first bunched electron beam E₁, and a secondinjector 31 is arranged to produce a second bunched electron beam E₂.Each injector 30, 31 is arranged to direct its respective bunchedinjected electron beam E₁, E₂, to steering unit 32. The steering unit 32is arranged to selectively direct one of the injected electron beams toan electron beam merging unit 33 (as illustrated this is the firstinjected electron beam E₁). The other of the injected electron beams isdirected to an electron beam dump 34 (as illustrated this is the secondinjected electron beam E₂). The electron beam dump 34 may, for example,include a body of water or a material with a high threshold forradioactive isotope generation by high energy electron impact, forexample aluminium (Al) with a threshold of approximately 15 MeV.

The steering unit 32 may be implemented in any appropriate way as willbe readily apparent to the skilled person. As an example, the steeringunit 32 may comprise steering magnets which can be controlled to steeran incoming injected electron beam E₁, E₂ in the direction of either thebeam dump 34 or the merging unit 33.

The steering unit 32 may be considered to operate in steering modes. Ina first steering mode the steering unit 32 directs the electron beamfrom the first injector 30 along a first path towards the beam dump 34.In a second steering mode the steering unit directs the electron beamfrom the first injector towards the merging unit 33.

The injected electron beam E₁ that is directed towards the merging unit33 provides an output electron beam. The merging unit 33 is arranged tomerge the injected electron beam provided by the steering unit 32 withan existing recirculating electron beam propagating within the freeelectron laser FEL and direct the merged electron beam E to the linearaccelerator 22. The merging unit 33 may be implemented in anyappropriate way as will be readily apparent to the skilled person. As anexample, the merging unit 33 may comprise a plurality of magnetsarranged to produce a magnetic field to direct the injected electronbeam E₁ from the steering unit 32 for merging with the recirculatingelectron beam. It will be appreciated that upon initialization of thefree electron laser FEL, there may not be a recirculating electron beamwith which to merge the electron beam provided by the steering unit 32.In this case, the merging unit 33 simply provides the electron beamprovided by the steering unit 32 to the linear accelerator 22.

The injector arrangement 21 of FIG. 4 allows one of the injectors 30, 31to be serviced while the other of the injectors 30, 31 is operational.For example, FIG. 4 shows the first injector 30 as operational. That is,the electron beam E₁ generated by the first injector 30 is directed bythe steering unit 32 to the merging unit 33 for provision to the linearaccelerator 22 (denoted by the representation of electron beam E₁ insolid line). As depicted in FIG. 4, the second injector 31 isnon-operational (denoted by the dotted line between the electron beam E₂and the merging unit 33 showing that the electron beam E₂ does notcontribute to the electron beam E).

In order to rapidly switch a non-operational injector to an operationalstate, the non-operational injector 31 may operate in a standby mode(standby injector) while the other injector 30 (operating injector)provides its bunched electron beam to the linear accelerator 22. Thestandby injector 31 may generate the same charge as the operatinginjector 30 but at a low duty cycle. The charge produced by the standbyinjector 31 may be provided to the dump 34 (as illustrated). This allowsthe non-operational injector 31 to quickly assume the role of theoperational injector in the event that the operational injector requiresmaintenance.

It will be appreciated that during maintenance, a non-operationalinjector may be in neither the operational mode or the standby mode, butmay be in an off state in which no charge is generated. Each of theinjectors 30, 31 may be shielded from each other in order to create asafe environment for servicing a non-operational injector. Each injectormay for example each be provided in a room having walls which provideshielding from radiation generated by the other injector. The rooms mayalso be shielded from other parts of the free electron laser FEL.

FIG. 5 schematically illustrates an injector arrangement 21 according toan alternative embodiment in which like components have been providedwith like reference numerals. In FIG. 5, first and second injectors 30,31 are each arranged to direct respective electron beams E₁, E₂ in adirection of a beam dump 34. Respective steering units 35, 36 aredisposed between each injector 30, 31 and the beam dump 34. Eachsteering unit 35, 36 may selectively direct a received electron beam toeither the beam dump 34 or to a merging unit 33. As such, while oneinjector 30, 31 operates in a standby (or completely non-operational)mode, the other injector 30, 31 may provide an electron beam to thelinear accelerator 22.

The steering units 35, 36 may be considered to operate in steeringmodes. In a first steering mode the first steering unit 35 directs theelectron beam from the first injector 30 along a first path towards thebeam dump 34. In a second steering mode the first steering unit 35directs the electron beam from the first injector 30 towards the mergingunit 33. In a third steering mode the second steering unit 36 directsthe electron beam from the second injector 31 along a third path towardsthe beam dump 34. In a fourth steering mode the second steering unit 36directs the electron beam from the second injector 31 towards themerging unit 33.

The arrangement of FIG. 5 may beneficially require less complicatedsteering mechanisms than the arrangement of FIG. 4. That is, beam dump34 may be positioned with respect to each steering unit 35, 36 such thatno adjustment to the path of electron beams E₁, E₂ is required to directthe electron beams E₁, E₂ to the beam dump 34. In this way, the steeringunits 35, 36 need only direct the electron beams E₁, E₂ to the mergingunit 33.

It may be desirable to ensure that a charge distribution within theelectron beam E provided to the merging unit 33 does not change whenswitching from one injector to a different injector (that istransitioning one injector from an operational state to a standby ornon-operational state while transitioning another injector from astandby or non-operational state to an operational state). Differencesin path lengths from each injector 30, 31 to the merging unit 33 maycause such changes in a distribution of charge within the electron beamprovided to the merging unit, for example as a result of differingamounts of expansion of the electron beams from each injector. It maytherefore be desirable to ensure that path lengths between all injectorsand the merging unit 33 are equal.

Where path lengths between injectors and the merging unit 33 are notequal, focusing elements may be provided. FIG. 6 schematicallyillustrates an alternative injector arrangement 21 in which likecomponents have been provided with like reference numerals. In thearrangement of FIG. 6, a first injector 30 is arranged to direct a firstelectron beam E₁ towards a first beam dump 34, while a second injector31 is arranged to direct a second electron beam E₂ towards a second beamdump 37. Respective steering units 38, 39 are disposed between eachinjector 30, 31 and its respective beam dump 34, 37. Each respectivesteering unit 38, 39 is operable to selectively divert a receivedelectron beam from its path towards a beam dump 34, 37 to a path towardsa merging unit 33 disposed adjacent to the steering unit 39.

The steering units 38, 39 may be considered to operate in steeringmodes. In a first steering mode the first steering unit 38 directs theelectron beam from the first injector 30 along a first path towards thebeam dump 34. In a second steering mode the first steering unit 38directs the electron beam from the first injector 30 towards the mergingunit 33. In a third steering mode the second steering unit 39 directsthe electron beam from the second injector 31 along a third path towardsthe beam dump 34. In a fourth steering mode the second steering unit 39directs the electron beam from the second injector 31 towards themerging unit 33.

A path length of electron bunches travelling between the first injector30 and the merger 33 is greater than a path length of electron bunchestravelling between the second injector 31 and the merger 33. As such, acharge distribution within the first electron beam E₁ at the mergingunit 33 may be different to a distribution of charge within the secondelectron beam E₂ at the merging unit 33. A focusing element 40 isprovided between the first steering unit 38 and the merging unit 33. Thefocusing element 40 may comprise, for example, a quadrupole magnetoperable to narrow or expand the first electron beam E₁ as necessary.The focusing element 40 may therefore adjust the focus of the firstelectron beam E₁ so that the charge distribution within the first andsecond electron beams E₁ and E₂ are the same at the merging unit 33.

It will be appreciated that while in the embodiment of FIG. 6, thefocusing element 40 is provided between the first steering unit 38 andthe merging unit 33, a focusing element may instead, or additionally beprovided between the second steering unit 39 and the merging unit 33. Afocusing element provided between the second steering unit 39 and themerging unit 33 may be operable to manipulate either or both of theelectron beams E₁, E₂. More generally, one or more focusing elements maybe provided at any point along the paths of the electron beams E₁, E₂.For example, in the embodiment of FIG. 4, if the first injector 31 isfurther from the steering unit 32 than the second injector 31, afocusing unit may be provided between the first injector 31 and thesteering unit 32.

It will further be appreciated that while in the schematic depiction ofFIG. 6 the first electron beam E₁ is shown as passing through the secondsteering unit 39, the first electron beam E₁ need not pass through thesecond steering unit 39. Further, where the first electron beam E₁ doespass through the second steering unit 39, the second steering unit 39may not need to actively steer the electron beam E₁.

In addition to path length, differences in angles through which electronbeams are bent between injectors and a merging unit can result indifferences in a distribution of charge between those electron beams atthe merging unit. FIG. 7 illustrates an alternative injector arrangement21 in which like components have been provided with like referencenumerals. In the arrangement of FIG. 7, a first injector 30 is arrangedto direct a first electron beam E₁ in the direction of a beam dump 34,while a second injector 31 is arranged to direct an electron beam E₂ inthe direction of a second beam dump 37. Respective steering units 38, 39are disposed between each injector 30, 31 and its respective beam dump34, 37. Each respective steering unit 38, 39 is operable to selectivelydivert a received electron beam from its path towards a beam dump 34, 37to a path towards a merging unit 33 disposed between the steering units38, 39. The arrangement of FIG. 7 provides one example of an injector 21in which the angles through which each electron beam E₁, E₂ is bentbetween its respective injector 30, 31 and the merging unit 33 areequal, thereby reducing a variance in charge distribution within theelectron beams E₁, E₂ that may be caused by differing bending angles.The merging unit 33 switches between merging the first injected electronbeam E₁ with the recirculating electron beam and merging the secondinjected electron beam E₂ with the recirculating electron beam. This maybe achieved using dipole magnets with switchable polarity, for examplein the manner described further below in connection with FIG. 11.

The steering units 38, 39 may be considered to operate in steeringmodes. In a first steering mode the first steering unit 38 directs theelectron beam from the first injector 30 along a first path towards thebeam dump 34. In a second steering mode the first steering unit 38directs the electron beam from the first injector 30 towards the mergingunit 33. In a third steering mode the second steering unit 39 directsthe electron beam from the second injector 31 along a third path towardsthe beam dump 34. In a fourth steering mode the second steering unit 39directs the electron beam from the second injector 31 towards themerging unit 33.

FIG. 8 schematically illustrates an alternative injector arrangement 21in which like components have been provided with like referencenumerals. The arrangement of FIG. 8 generally corresponds to thearrangement of FIG. 6, having the same components in a generally similarlayout. In the injector arrangement of FIG. 6, however, the direction atwhich the electron beams E₁, E₂ are emitted by the respective injectors30, 31 is depicted as being substantially perpendicular to the directionof the electron beam E as it enters the merging unit 33. That is, in thearrangement of FIG. 6, the steering units 38, 39 are depicted as bendingelectron beams E₁, E₂ through an angle of 90 degrees. In contrast, inthe arrangement of FIG. 8, the injectors 30, 31 and their respectivebeam dumps 34, 37 are each arranged at an angle with respect to thedirection of propagation of the injected electron beam E₁, E₂ as itenters the merging unit 33 such that an angle through which the steeringunits 38, 39 need bend each electron beam E₁, E₂ to direct that electronbeam to the merging unit 33 is less than 90 degrees. Similarly, in thearrangement of FIG. 8, the merging unit 33 is arranged to bend theinjected electron beams E₁, E₂ through an angle of less than 90 degrees.

In the arrangement of FIG. 8, therefore, the total angle through whicheach electron beam E₁, E₂ is bent is reduced compared to the arrangementof FIG. 6, thereby reducing detrimental effects of variance in chargedistribution within the electron beams.

While it is generally described above that one injector is operationalat any time, in general more than one injector may be operational at thesame time. For example, in the arrangement of FIG. 5, each injector 30,31 may be operational at the same time. That is, the steering units 35,36 may be arranged to simultaneously direct both of the electron beamsE₁, E₂ to the merging unit 33, the electron beams E₁, E₂ togetherproviding the electron beam E to be provided to the linear accelerator22. Where both electron beams E₁, E₂ are provided to the merging unit 33each injector 30, 31 may operate at a reduced repetition rate (that ismay emit fewer electron bunches in a given period of time). For example,each of the injectors 30, 31 may operate at a repetition rate that ishalf that of the repetition rate of one injector operating alone. Inthis case, the electron beam E may comprise electron bunches from thefirst injector 30 interleaved with electron bunches from the secondinjector 31.

Although in described embodiments each injector arrangement comprisestwo injectors 30, 31, it will be appreciated that additional injectorsmay be provided (with the provision of corresponding additional steeringarrangements where necessary). Additionally, although in the describedembodiments each injector arrangement provides an electron beam to amerging unit (i.e. in an FEL configuration that may be referred to as anEnergy Recovery LINAC (ERL) FEL), it is to be understood that whereenergy recovery is not used, the electron beams generated by theinjector arrangement are not provided to a merger, but are provideddirectly to the LINAC 22 without being merged with an existing electronbeam. In this case, an additional steering unit may be required todirect the electron beam E to the LINAC. Alternatively, the injectorarrangement may be arranged such that the electron beam E is providedfrom the injector arrangement without the use of additional steeringunits.

As described above, each injector may comprise a photocathode arrangedto generate electrons. The photocathode of each injector may be arrangedto receive a beam of radiation from a radiation source such as a laser(referred to herein as a photocathode drive laser).

The photocathode of each injector may be held at a high voltage by usinga voltage source. For example, a photocathode of an injector may be heldat a voltage of approximately several hundred kilovolts. Photons of thelaser beam are absorbed by the photocathode and may excite electrons inthe photocathode to higher energy states. Some electrons in thephotocathode may be excited to a high enough energy state that they areemitted from the photocathode. The high voltage of the photocathode isnegative and thus serves to accelerate electrons which are emitted fromthe photocathode away from the photocathode, thereby forming a beam ofelectrons.

The laser beam provided by the photocathode drive laser may be pulsedsuch that electrons are emitted from the photocathode in bunches whichcorrespond to the pulses of the laser beam. The photocathode drive lasermay, for example, be a picosecond laser and thus pulses in the laserbeam may have a duration of approximately a few picoseconds. The voltageof the photocathode may be a DC voltage or an AC voltage. In embodimentsin which the voltage of the photocathode is an AC voltage the frequencyand phase of the photocathode voltage may be matched with pulses of thelaser beam such that pulses of the laser beam coincide with peaks in thevoltage of the photocathode.

In some embodiments, a single photocathode drive laser may supply alaser beam to the photocathodes of multiple injectors. An exampleembodiment is schematically illustrated in FIG. 9, in which twoinjectors 30, 31 are driven by a single photocathode drive laser 50. Thephotocathode drive laser 50 is arranged to emit a pulsed laser beam 51to a beam splitter 52. The beam splitter 52 is arranged to split thelaser beam 51 into two pulsed laser beams 53, 54 directed to theinjectors 30, 31 respectively. The pulsed laser beams 53, 54 enter theinjectors 30, 31 through respective windows 55, 56 provided in a housingof the injectors 30, 31 and are incident upon respective mirrors 57, 58.

The mirror 57 of the first injector 30 is arranged to direct the pulsedlaser beam 53 onto a first photocathode 59, while the mirror 58 of thesecond injector 31 is arranged to direct the pulsed laser beam 54 onto asecond photocathode 60 causing the photocathodes 59, 60 to emitrespective electron bunches E₁, E₂.

In order to operate one of the injectors 30, 31 in a standby mode, or toallow one of the injectors 30, 31 to be non-operational (e.g. formaintenance), the beam splitter 52 may be operable to selectivelyprevent laser radiation from being directed to either of the injectors30, 31. The beam splitter 52 may be operable to independently vary thefrequency with which pulsed laser beams are provided to the respectiveinjectors 30, 31. For example, where the first injector 30 isoperational and the second injector 31 is to be in a standby mode, thefrequency with which pulsed radiation beams are provided to the secondinjector 31 may be lower than the frequency with which pulsed radiationbeams are provided to the first injector 30.

Alternatively, the beam splitter 52 may always provide the pulsed laserradiation beams 53, 54 to both injectors equally. In this case, aninjector may be placed into a standby mode by adjusting a voltageapplied to the photocathode of the injector. For example, a voltageapplied to a photocathode of a standby injector may be brought high lessfrequently than a voltage applied to a photocathode of an operationalinjector (thereby reducing the duty cycle of the standby injector).

It will be appreciated that FIG. 9 is merely schematic and that eachinjector may comprise more components than are illustrated. For examplethe photocathode of each injector may be housed within a vacuum chamber,and each injector may comprise an accelerating electric field.

FIG. 10 schematically shows a free electron laser FEL which comprises aninjector arrangement 121, a linear accelerator 122 and an undulator 124.The injector arrangement 121 is provided in a first room 180 and thelinear accelerator arrangement 122 and undulator 124 are provided in asecond room 181. An EUV radiation beam B is output from the undulator124 and may be provided to lithographic apparatuses (e.g. in the mannerdescribed above in connection with FIG. 1).

The injector arrangement 121 comprises a first injector 130 and a secondinjector 131. The first injector 130 is located in a first room 178 andthe second injector 131 is located in a second room 179. Each injector130, 131 comprises its own electron source such as, for example, aphotocathode, and an accelerating electric field (e.g. as shown in FIG.12). The accelerating electric field accelerates electrons generated bythe electron source such that they leave the injector 130, 131 with anenergy of for example around 10 MeV. The injectors 130, 131 generateelectron beams E₁, E₂ which are directed towards a merging unit 133. Themerging unit 133 merges the electron beams E₁, E₂ with a recirculatingelectron beam E_(IR) (the origin of the recirculated electron beamE_(IR) is described further below). Each injected electron beam E₁, E₂subtends an angle a with respect to the recirculating electron beamE_(IR) when it merges with the recirculating beam at the merging unit133. The angle α may for example be around 30° or less, and may forexample be around 15° or less.

In practice, during normal operation one of the injectors 130, 131 maybe offline, e.g. operating in a standby mode or switched off to allowfor routine maintenance. Thus, the recirculating electron beam E_(IR) ismerged with either the electron beam E₁ generated by the first injector130 or the electron beam E₂ generated by the second injector 131. Forease of terminology the injected electron beam after the merging unit133 is labelled as injected electron beam E (instead of injectedelectron beam E₁ or E₂).

A linear accelerator 150 forms part of the injector arrangement 121. Thelinear accelerator 150 accelerates the electrons of the injectedelectron beam E and increases their energy by at least 20 MeV. Since theelectrons enter the linear accelerator 150 with an energy of around 10MeV, they leave the linear accelerator with an energy of 30 MeV or more.The injector arrangement thus provides an electron beam E with an energyof 30 MeV or more. This electron beam E passes through an opening (notshown) out of the first room 180 and into the second room 181.

In the second room 181 the linear accelerator 122 accelerates theelectrons of the electron beam E. The energy provided to the electronsby the linear accelerator 122 is significantly greater than the energyprovided by the linear accelerator 150 of the injector arrangement 121.The energy provided by the linear accelerator 122 may for example bearound 100 MeV or more). The accelerated electron beam E passes from thelinear accelerator 122 to the undulator 124. In the undulator aradiation beam B is generated by the electron beam E in the mannerdescribed further above.

The linear accelerator 122 in the second room 181 may be referred to asthe main linear accelerator 122 (or the second linear accelerator), andthe linear accelerator 150 in the first room 180 may be referred to asthe injector linear accelerator 150. The injector linear accelerator 150and the main linear accelerator 122 may in an alternative arrangement beprovided in the same room.

Accelerating the electron beam E after generation by the injector 130,131 and before the electron beam travels to the main linear accelerator122 is advantageous because it significantly improves the quality ofelectron bunches which are received by the main linear accelerator. Theelectron beam E may travel a substantial distance (e.g. in excess of 10m) when travelling from the injector arrangement 121 to the main linearaccelerator 122. If the electrons of the electron beam E were to have anenergy of around 10 MeV, such as may be expected to be provided by aninjector 130, 131, then significant degradation of the quality ofelectron bunches in the electron beam would occur as the electronstravelled to the main accelerator 122. The term ‘quality’ in thiscontext may be interpreted as referring to the compactness of theelectron bunch and the spread of electron energies within an electronbunch. The degradation occurs due to space charge effects. These spacecharge effects, which are examples of micro-bunch instability effects,are unavoidable. Accelerating the electron beam E using the injectorlinear accelerator 150 significantly increases the Lorentz factor of theelectrons, and as a result the electrons have an increased mass as theytravel from the injector arrangement 121 to the main linear accelerator122. The increased mass of the electrons reduces the bunch degradationcaused by space charge effects because the acceleration applied to theelectrons due to space charge forces is reduced. The quality of theelectron bunches is accordingly increased.

In an embodiment, if the energy of the electrons is increased from 10MeV to 30 MeV or more then the mass of the electrons is approximatelytripled or more than tripled. Electron beam bunch degradation iscorrespondingly reduced by two thirds or more. Some micro-bunchinstability effects scale are reduced nonlinearly with respect to theLorentz factor γ, and may for example reduce with a factor of γ² or witha factor of γ³. These micro-bunch instability effects are thus reducedvery substantially as the energy of the electrons is increased.

The injector linear accelerator 150 may provide significantly more than20 MeV of energy to the electrons. It may for example provide 30 MeV ormore. It may for example provide 50 MeV or more, or 60 MeV or more. Theinjector linear accelerator 150 may for example be provided as a modulewhich is configured to provide 50 MeV or more, or 60 MeV or more.Alternatively, the injector linear accelerator 150 may be provided ashalf of such a module, which may be configured to provide 20 MeV ormore, or 30 MeV or more. Providing more energy to the electron beamfurther reduces micro-bunch instability effects and thus furtherincreases the quality of electron bunches that are received by the mainaccelerator 122.

It might be considered that instead of increasing the energy of theelectrons in the electron beam E, the length of the path from theinjectors to the main linear accelerator could simply be reduced.However, doing this may be problematic in practice, and large bendingangles, e.g. in excess of 45° over short distances may be required. Thismay be the case for example when the injectors 130, 131 are located indifferent rooms 178, 179 from the main linear accelerator (this may bedesirable to allow maintenance of an injector during operation of thefree electron laser FEL). Large bending angles experienced by theelectron beam E will cause the electrons to emit coherent synchrotronradiation. Coherent synchrotron radiation emitted by electrons at thefront of an electron bunch will interact with electrons at the back ofan electron bunch. The coherent synchrotron radiation thus disturbs theelectron bunch and degrades its quality. The emission of coherentsynchrotron radiation and interaction of that radiation with theelectron bunch is another example of a micro-bunch instability effect.

When an embodiment of the invention is used, space charge instabilityeffects experienced by the electrons of the electron beam E are verysubstantially reduced. This allows the length of the path from theinjector arrangement 121 to the main linear accelerator 122 to beincreased whilst incurring only a small reduction in the quality ofelectron bunches in the electron beam as a result of the lengthincrease. Increasing the length of the path may allow the electron beamE to be bent more gradually (i.e. a longer path length is available toachieve a given change of direction of the electron beam). The change ofelectron beam direction may for example be achieved using a combinationof dipole and quadrupole magnets. Providing a longer path length toaccommodate the dipole and quadrupole magnets may allow them to bearranged such that they cause less synchrotron radiation to be emittedduring the change of electron beam direction (compared with the amountof synchrotron radiation emitted during a change of electron beamdirection when a shorter path length is available to accommodate themagnets).

Thus, by increasing the energy of the electrons in the electron beam andthereby increasing the Lorentz factor (and the mass) of the electrons,micro-bunch instability effects are reduced. The quality of electronbunches in the electron beam received by the main accelerator 122 isthereby improved.

The injector linear accelerator 150 is an energy recovery linearaccelerator. That is, the injector linear accelerator 150 transfersenergy from the recirculating electron beam E_(IR) to the injectedelectron beam E. The recirculating electron beam E_(IR) enters theinjector linear accelerator 150 with a phase difference of around 180degrees relative to accelerating fields in the injector linearaccelerator (e.g. radio frequency fields). The phase difference betweenthe electron bunches and the accelerating fields in the injector linearaccelerator 150 causes the electrons of the recirculating electron beamE_(IR) to be decelerated by the fields. The decelerating electrons passsome of their energy back to the fields in the injector linearaccelerator 150 thereby increasing the strength of the fields whichaccelerate the injected electron beam E. In this way energy istransferred from the recirculating electron beam E_(IR) to the injectedelectron beam E.

In an embodiment the recirculating electron beam E_(IR) has an energy of30 MeV, and the injector linear accelerator 150 transfers 20 MeV ofenergy from the recirculating electron beam to the injected electronbeam E. Thus, an output electron beam E with an energy of 30 MeV and arecirculated electron beam E_(IR) with an energy of 10 MeV are providedfrom the injector linear accelerator 150. The recirculated electron beamE_(IR) is separated from the electron beam E by a demerging unit 134 andis directed to a beam dump 151.

Because the injector linear accelerator 150 is an energy recovery linearaccelerator and transfers energy from the recirculated electron beamE_(IR) to the injected electron beam E, it uses far less energy thanwould be the case if the linear accelerator was not an energy recoverylinear accelerator. The injector linear accelerator 150 may have abalanced cavity load which is close to zero. That is, the current in theinjected electron beam E may substantially match the current in therecirculated electron beam E_(IR), and the energy extracted from therecirculated electron beam E_(IR) may be almost the same as the energygiven to the injected electron beam E. In practice, the amount of energygiven to the injected electron beam E might be slightly higher than theenergy extracted from the recirculated electron beam E_(IR), in whichcase some energy is provided to the injector linear accelerator 150 tomake up this difference. In general, the energy of the recirculatedelectron beam E_(IR) when it leaves the injector linear accelerator 150will substantially correspond with the energy of the electron beam E₁provided by the first injector 130 (or equivalently the electron beam E₂provided by the second injector 131).

As mentioned above, the injectors 130, 131 each include an acceleratingelectric field which accelerates the electrons before they reach themerging unit 133. The accelerating electric field is provided by alinear accelerator which uses the same operating principle as theinjector linear accelerator 150 and the main linear accelerator 122,i.e. cavities are provided with a radio-frequency (RF) field whichaccelerates the electrons. However, an important difference between theacceleration provided within the injectors 130, 131 and subsequentacceleration is that that injection is not provided by an energyrecovery linear accelerator. Thus, all of the energy required toaccelerate the electrons before they reach the merging unit 133 must beprovided to the injectors 130, 131 (none of the energy is recoveredenergy). For example, to accelerate the electrons to 10 MeV around 300kW of power is needed. If the electrons were to be accelerated forexample to 30 MeV by an injector 130, 131 then this would require around900 kW of power. A disadvantage of providing such high power is thatcryogenic cooling of the injector 130, 131 may become problematic. Inaddition, complications may arise when switching an injector 130, 131 onand off due to the magnitude of the load connected across the powersupply.

Using the energy recovery injector linear accelerator 150 to acceleratethe electrons to 30 MeV (or some other energy) avoids the above problemsbecause the energy used to accelerate the electrons is recovered fromthe recirculated electron beam E_(IR). An additional benefit ofinjecting the electron beam with a relatively low energy (e.g. 10 MeV orless) and then using the energy recovery injector linear accelerator 150to accelerate the electron beam is that the recirculated electron beamE_(IR) has an energy of 10 MeV or less after it has passed through theinjector linear accelerator. This is energy is sufficiently low to avoidinducing radioactivity in the beam dump 151. If the injected electronbeam had a significantly higher energy (e.g. 20 MeV) then an electrondecelerating unit would need to be added before the beam dump 151 inorder to avoid inducing radioactivity in the beam dump.

The main linear accelerator 122 is also an energy recovery linearaccelerator. Energy recovery in the main linear accelerator 122 works inthe same manner as energy recovery in the injector linear accelerator150. After leaving the undulator 124 the electron beam E is recirculatedthrough the main linear accelerator 122 with a phase difference ofaround 180 degrees. The electron beam then enters the injectorarrangement 121 as the recirculating electron beam E_(IR) which mergeswith an injected electron beam E₁, E₂.

The electron bunches of the electron beam E may be provided as sequencesof bunches with gaps being provided between the sequences. The gaps maybe referred to as clearing gaps and are longer than the separationbetween adjacent electron bunches of an electron bunch sequence. Ionsare produced from residual gas in the electron beam path throughcollisional ionization. The ions are positively charged, and the rate ofgeneration of the ions is such that over time the electron beam E wouldbe neutralized (e.g. ion charge matches electron charge per meter ofelectron beam) if the ions were not removed. The clearing gaps in theelectron beam E allow ions to drift away from the electron beam path,thereby preventing or reducing the accumulation of trapped ions. Thisdrifting of ions away from the electron beam path may take place at anypoint along the beam path. Extraction electrodes may be provided whichact to increase the speed at which ions drift away from the electronbeam path.

As may be understood from considering FIG. 10, at some positions in thefree electron laser the electron beam E and the recirculating electronbeam E_(IR) co-propagate with each other. This occurs between themerging unit 133 and the demerging unit 134, with the beamsco-propagating through the injector linear accelerator 150 (it is thisco-propagation which allows the energy recovery to happen in the linearaccelerator). Similarly, the electron beam E and the recirculatingelectron beam E_(IR) also co-propagate through the main linearaccelerator 122.

In order for clearing gaps to be effective at locations where theelectron beam E and the recirculating electron beam E_(IR) co-propagate,the clearing gaps in the electron beam E should be synchronized with theclearing gaps in the recirculating electron beam E_(IR). In addition toallowing ions to drift out of the electron beam path, synchronizing theclearing gaps as they pass through the injector linear accelerator 150and the main linear accelerator 122 also provides the advantage thatenergy recovery operation of the accelerators is not disturbed (if aclearing gap were present in the decelerating beam without acorresponding clearing gap in the accelerating beam then the acceleratorwould cause an unwanted fluctuation in the energy of the acceleratedelectron beam).

In an embodiment, in order to allow synchronization of clearing gaps,the electron beam path length of the two loops shown in FIG. 10 may beequal to each other. The first loop may be measured from the mergingunit 133, through the injector linear accelerator 150 and the mainlinear accelerator 122 back to the merging unit (without passing throughthe undulator 124). The second loop may be measured from entrance of themain linear accelerator 122, through the main linear accelerator and theundulator 124 and back to the entrance of the main linear accelerator122. The rate at which the clearing gaps are generated may correspondwith the time taken for electrons to travel around one of the loops (thetravel time around the first loop will be the same as the travel timearound the second loop). This will provide synchronization of theclearing gaps in the electron beam E and the recirculating electron beamE_(IR), as a result of which ion clearance can take place throughout theelectron beam path (including in the injector linear accelerator 150 andthe main linear accelerator 122). In general, clearing gaps in theinjected electron beam may be synchronized with clearing gaps in therecirculating electron beam.

Although FIG. 10 schematically shows the injector arrangement 121 asbeing in the same plane as the main linear accelerator 122 and theundulator 124, it is not necessary that this is the case. The injectorarrangement 121 may be provided in a different plane. For example, theroom 180 in which the injector arrangement is provided may be above orbelow the room 181 in which the main linear accelerator 122 and theundulator 124 are provided.

In general, each room 180-183 of the free electron laser FEL may haveradiation shielding walls, floors and ceilings, such that radiation fromoutside that room is not incident upon an operator inside that room (andvice versa). This may allow for example an operator to repair oneinjector 130 whilst the other injector 131 is operating and other partsof the free electron laser FEL are operating.

FIG. 11 shows schematically in more detail the merging unit 133 togetherwith the first and second injectors 130, 131 of the injector arrangement121. In FIG. 11 the second injector 131 is operational and the firstinjector 130 is switched off (e.g. to allow for maintenance) or is in astandby mode. The merging unit 133 combines the injected electron beamE₂ with the recirculating electron beam E_(IR) such that when these twoelectron beams leave the merging unit they propagate together in acollinear manner.

The merging unit 133 comprises dipole magnets and quadrupole magnets. Inthis embodiment the dipole magnets and the quadrupole magnets areelectromagnets (although the dipole magnets and/or quadrupole magnetsmay be permanent magnets). The dipole magnets are schematicallyrepresented by squares containing discs 161, 162, 170-173, 181, 182. Thedipole magnets change the direction of propagation of the electron beams(the point at which a change of direction occurs is representedschematically by a disc). Quadrupole magnets are indicated schematicallyby rectangles which do not contain discs 163, 183, 175. The quadrupolemagnets act to keep the electron beams focused, i.e. to prevent unwanteddivergence of the electron beam.

The injected electron beam E₂ and the recirculating electron beam E_(IR)are combined by combining dipole magnet 173. As is schematicallyillustrated, the injected electron beam E₂ and the recirculatingelectron beam E_(IR) have different orientations relative to an axis Aof the injector arrangement (indicated by a dotted line). In thisembodiment, the injected electron beam E₂ subtends an angle of forexample around 15° relative to the axis A as it enters the combiningdipole magnet 173. The recirculating electron beam E_(IR) subtends anangle of for example around 2° relative to the axis A as it enters thecombining dipole magnet 173. The injected electron beam has an energy ofaround 10 MeV, and the recirculating electron beam E_(IR) has an energyof around 80 MeV in this example.

The combining dipole magnet 173 bends both of the electron beams E₂,E_(IR) to the right as they pass through the combining dipole magnet.The bending angle which is applied to the electron beams E₂, E_(IR) bythe combining dipole magnet 173 is inversely proportional to the energyof the electron beams. The injected electron beam E₂, which has anenergy of 10 MeV, is bent through an angle of around 15° such that whenit exits the combining dipole magnet 173 it propagates in the directionof the axis A. The recirculating electron beam E_(IR), which has aconsiderably higher energy of 80 MeV, is bent through an angle of around2°. The bending angle of 2° is such that when the recirculating electronbeam E_(IR) exits the combining dipole magnet 173 it also propagates inthe direction of the axis A. The spatial position of the injectedelectron beam E₂ on entering the combining dipole magnet 173 and thespatial position of the recirculating electron beam E_(IR) on enteringthe dipole magnet are selected such that they both have the same spatialposition on leaving the combining dipole magnet 173. The combiningdipole magnet 173 thus combines the two electron beams such that onexiting the combining dipole magnet they both propagate along the axis A(they are collinear with each other).

The other dipole magnets 161, 162, 170-172 shown in FIG. 11 areconfigured to deliver the electron beams E₂ E_(IR) to the combiningdipole magnet 173 at angles and spatial positions relative to the axis Awhich are such that the electron beams will both propagate along thecentral axis when they leave the combining dipole magnet 173 (i.e. theywill be collinear). Since the energies of the electron beams E₂, E_(IR)are determined during design of the free electron laser, the generalconfiguration of the dipole magnets may be selected accordingly when thefree electron laser is being designed. The beam bending angles providedby the dipole magnets 161, 162, 170-173 may be tuned during installationof the injector arrangement 121 in order to provide beam alignment.

In addition to delivering the electron beams E₂ E_(IR) to the combiningdipole magnet 173 at desired angles and spatial positions relative tothe axis A, the dipole magnets dipole magnets 161, 162, 170-172 may alsobe configured to maintain (or substantially maintain) electron bunchquality in the electron beams. Thus, the

A pair of dipole magnets 161, 162 are provided in the path of theinjected electron beam E₂. The first dipole magnet 161 is arranged tobend the injected electron beam E₂ to the right, and the second dipolemagnet 162 is arranged to bend the injected electron beam E₂ to theleft. After passing through the pair of dipole magnets 161, 162 theinjected electron beam E₂ then passes through the combining dipolemagnet 173, which bends the injected electron beam to the right. Theinjected electron beam E₂ thus passes through three dipole magnets 161,162, 173. The injected electron beam E2 also passes through quadrupolemagnets, which are provided before the first dipole magnet 161, betweenthe first and second dipole magnets 161, 162 and after the second dipolemagnet 162.

The dipole magnets 161, 162 are configured to deliver the injectedelectron beam E₂ to the combining dipole magnet 173 at a desired anglerelative to the axis A (the desired angle may for example be 15°).Quadrupole magnets 163 are provided before the first dipole magnet 161,between the first and second dipole magnets 161, 162 and after thesecond dipole magnet 162. The quadrupole magnets 163 keep the injectedelectron beam E₂ focused, i.e. prevent unwanted divergence of theinjected electron beam. In combination, the three dipole magnets 161,162, 173 and three quadrupole magnets 163 provide bending of theinjected electron beam E₂ which is substantially achromatic, i.e. theposition and direction of the electron beam after the combining dipolemagnet 173 is independent of the energy of the injected electron beamE₂. The dipole magnets 161, 162, 173 and quadrupole magnets 163 alsoprovide bending of the injected electron beam E₂ which is substantiallyisochronous, i.e. all energies of electrons travel along the same pathlength. Some tuning of the dipole magnets 161, 162, 173 and quadrupolemagnets 163 may be performed during installation in order to obtain anelectron beam with a desired bunch quality. The tuning may for exampletake into account the quality of the electron bunches after the injectorlinear accelerator 150. In some instances, the best electron bunchquality after the injector linear accelerator 150 may be achieved bydeliberately introducing, for example, a small amount of chromaticityinto the injected electron beam E2 using the dipole magnets 161, 162,173 and quadrupole magnets 163. In general, the dipole magnets 161, 162,173 and quadrupole magnets 163 may be tuned to provide electron buncheswith a desired quality after the linear accelerator 150.

The injected electron beam E₂ may travel along a solenoid (not shown)before it reaches the first quadrupole magnet 163. The solenoid may passthrough a wall of a room 179 in which the second injector 131 is located(see FIG. 10).

The recirculated electron beam E_(IR) passes through four dipole magnets170-173. These dipole magnets 170-173 have reversible polarity. That is,the bending angle which is applied to the recirculated electron beamE_(IR) by each dipole magnet 170-173 may be reversed. This is achievedby switching the direction of current which flows through the dipolemagnets 170-173, thereby swapping the B-field direction of those dipolemagnets.

When the second injector 131 is operating and the first injector 130 isswitched off (or in standby mode), the dipole magnets 170-173 areconfigured such that the recirculating electron beam E_(IR) follows thepath indicated schematically by the solid line in FIG. 11. That is, thefirst dipole magnet 170 bends the recirculating electron beam E_(IR) tothe right, the second dipole magnet 171 bends the recirculating electronbeam E_(IR) to the left, and the third dipole magnet 172 bends therecirculating beam to the right. In this example, the dipole magnets170-172 deliver the recirculating electron beam E_(IR) to the combiningdipole magnet 173 at an angle of around 2° relative to the axis A. Thedipole magnets 170-173 are arranged as a chicane. The chicane deliversthe recirculating electron beam E_(IR) to the combining dipole magnet173 at a desired angle such that the recirculating electron beam isdirected along the axis A when it leaves the combining dipole magnet.

As noted above, the angles of incidence of the injected electron beam E₂and the recirculating electron beam E_(IR), and their respectiveenergies, are such that they both propagate along the axis A when theyleave the combining dipole magnet 173. The injected electron beam E₂ andthe recirculating electron beam E_(IR) pass through quadrupole magnets175 and then travel to the injector linear accelerator 150 (see FIG.10).

It may be desired to switch off the second injector 131 and use thefirst injector 130 to provide an electron beam E₁ for the free electronlaser (i.e. switch between a second mode of operation and a first modeof operation). Where this is the case, the polarities of the dipolemagnets 170-173 which act upon the recirculating electron beam E_(IR)are all switched. This may be achieved by switching the direction ofcurrent flowing through the dipole magnets 170-173. The recirculatingelectron beam E_(IR) then follows the beam path indicated by the dashedline. This is effectively a mirror image of the path followed by therecirculating electron beam E_(IR) when the second injector 131 wasbeing used (reflected about the axis A). Thus, the recirculatingelectron beam E_(IR) is bent to the left by the first dipole magnet 170,is then bent to the right by the second dipole magnet 171, and is thenbent to the left by the third dipole magnet 172. The recirculatingelectron beam is thereby delivered to the combining dipole magnet 173 atan angle of around 2°, but from an opposite side of the axis A.

Similarly, the injected electron beam E₁ follows a path which is alsoindicated by a dashed line. The injected electron beam E₁ passes throughtwo dipole magnets 181, 182 and three quadrupole magnets 183. Thesemagnets operate in a manner which corresponds with that described abovein relation to the other injected electron beam E₂, and deliver theinjected electron beam E₁ to the combining dipole magnet 173 at an angleof around 15°, but from an opposite side of the axis A.

The energies and incidence angles of the injected electron beam E₁ andthe recirculating electron beam E_(IR) are such that the combiningdipole magnet 173 applies different degrees of bending to the electronbeams, and they both propagate collinearly along the axis A when theyleave the combining dipole magnet.

The above description refers to incidence angles of around 15° for theinjected electron beams E₁, E₂ and incidence angles of around 2° for therecirculating electron beam E_(IR). However, it will be appreciated thatany suitable angles may be used (e.g. incidence angles of up to around30° for the injected electron beams E₁, E₂, and incidence angles of upto around 4° for the recirculating electron beam E_(IR)). As notedabove, the degree of bending which is applied by the combining dipolemagnet 173 is inversely proportional to the energy of the electron beam.Thus, when configuring the injector arrangement 121 the configurationsof the dipole magnets 161, 162, 170-173, 181, 182 may be selected usingthe electron beam energies that will be present when the injectorarrangement 121 is operational (the energies of the electron beams willbe known in advance).

Referring again to FIG. 10, it may be seen that the injected electronbeam E₂ appears to cross the recirculating electron beam E_(IR) upstreamof the merging unit 133. In practice the electron beams do notintersect, but instead the injected electron beam E₂ passes above therecirculating electron beam E_(IR). Dipole magnets are used to move therecirculating electron beam E_(IR) upwards after it has passedunderneath the injected electron beam E₂, such that both beams arepropagating in the same plane before they enter the merging unit 133.The plane of injected electron beam E₂ and the recirculating electronbeam E_(IR) may correspond with the plane of the merged electron beam E,E_(IR) after the merging unit 133. The plane may for example besubstantially horizontal. In an alternative arrangement the injectedelectron beam E₂ may pass beneath the recirculating electron beam E_(IR)before the merging unit 133.

Switching between the first mode of operation, in which the injectedelectron beam E₁ provided from the first injector 130 is combined withthe recirculating electron beam E_(IR), to the second mode of operationin which the injected electron beam E₂ provided from the second injector131 is combined with the recirculating electron beam, may be controlledby a controller (not shown). The controller may comprise a processor.The controller may switch the polarity of the dipole magnets 170-173 ofthe chicane by switching the direction of current provided to the dipolemagnets.

FIG. 12 is a schematic depiction of an embodiment of an injector 230.The injector 230 comprises an electron gun 231 (which may be consideredto be an electron source), an electron booster 233 and a steering unit240. The electron gun 231 comprises a support structure 242 which isarranged to support a photocathode 243 inside a vacuum chamber 232. Itshould be appreciated that in industry an injector 230 may be soldwithout a photocathode 243 which may be considered as a replaceable partfor use in the injector 230.

The electron gun 231 is configured to receive a beam of radiation 241from a radiation source 235. The radiation source 235 may, for example,comprise a laser 235 which emits a laser beam 241. The laser 235 may bereferred to as a photocathode drive laser. The laser beam 241 isdirected through a laser beam adjustment unit 238 and into the vacuumchamber 232 through a window 237. The laser beam 241 is reflected by amirror 239 such that it is incident on the photocathode 243. The mirror239 may, for example, be metallised and connected to ground in order toprevent the mirror 239 from becoming electrically charged.

The laser beam adjustment unit 238, the window 237 and the mirror 239may all be considered to be components of a beam delivery system whichdirects the laser beam 241 onto a region of the photocathode 243. Inother embodiments the beam delivery system may comprise more or fewercomponents than the laser beam adjustment unit 238, the window 237 andthe mirror 239 and may comprise other optical components. The beamdelivery system may comprise any components suitable for directing thelaser beam 241 onto a region of the photocathode 243. For example, insome embodiments the beam delivery system may consist only of a supportconfigured to support a laser 235 such that a laser beam 241, emitted bythe laser 235, is directed onto a region of the photocathode 243.

The photocathode 243 may be held at a high voltage by using a voltagesource (not shown) which may form part of the electron gun 232 or may beseparate from the electron gun 232. For example, the photocathode 243may be held at a voltage of approximately several hundred kilovolts.Photons of the laser beam 241 are absorbed by the photocathode 243 andmay excite electrons in the photocathode 243 to higher energy states.Some electrons in the photocathode 243 may be excited to a high enoughenergy state that they are emitted from the photocathode 243. The highvoltage of the photocathode 243 is negative and thus serves toaccelerate electrons which are emitted from the photocathode 243 awayfrom the photocathode 243, thereby forming a beam of electrons E.

As mentioned further above, the laser beam 241 may be pulsed such thatelectrons are emitted from the photocathode 243 in bunches whichcorrespond to the pulses of the laser beam 241. The electron beam E istherefore a bunched electron beam. The laser 235 may, for example, be apicosecond laser and thus pulses in the laser beam 241 may have aduration of approximately a few picoseconds. The voltage of thephotocathode 243 may be a DC voltage or an AC voltage. In embodiments inwhich the voltage of the photocathode 243 is an AC voltage the frequencyand phase of the photocathode voltage may be matched with pulses of thelaser beam 241 such that pulses of the laser beam 241 coincide withpeaks in the voltage of the photocathode 243.

The amount of radiation emitted from the free electron laser FEL is atleast in part dependent on a peak current of the electron beam E in theundulator 24. In order to increase the peak current of an electron beamE in the undulator 24 and therefore increase the amount of radiationwhich is emitted from the free electron laser FEL, it may be desirableto increase the peak current of electron bunches which are emitted fromthe photocathode 243. For example, it may be desirable for thephotocathode 243 to emit electron bunches having a peak currentexceeding 1 milliamp.

The number of electrons which are emitted by the photocathode 243 perphoton from the laser beam 241 is known as the photocathode's quantumefficiency. It may be desirable for the photocathode 243 to comprise amaterial having a high quantum efficiency such that an electron beam Ehaving a large peak current (e.g. a peak current of greater than 1milliamp) is emitted from the photocathode 243 for a given number ofphotons of the laser beam 241. The photocathode 243 may, for example,comprise one or more alkali metals and may comprise a compoundcontaining one or more alkali metals and antimony. For example, the filmof material 63 may comprise sodium potassium antimonide. Such aphotocathode 243 may, for example, have a quantum efficiency of a fewpercent. For example the photocathode 243 may have a quantum efficiencyof approximately 5% (this may be considered to be a high quantumefficiency).

The vacuum chamber 232 extends from the electron gun 231 and through theelectron booster 233, thus forming a beam passage 234 through which theelectron beam E travels. The beam passage 234 extends about an axis 245.In an embodiment in which the electron beam E is delivered directly to alinear accelerator which does not use energy recovery, the axis 245 maycorrespond with the desired path of the electron beam E through thelinear accelerator 22, and may be the axis about which the electronsfollow an oscillating path in the undulator 24 (as was described above).In an embodiment in which the electron beam E is delivered via a mergingunit to join a recirculating electron beam, the axis 245 may correspondwith a desired path of the electron beam E on leaving the injector 230(the desired path being such that the electron beam will be delivered bydipole magnets to the merging unit with a desired incidence angle).

The axis 245 may coincide with the geometric centre of the beam passage234 and/or the geometric centre of the photocathode 243. In alternativeembodiments the axis 245 may be separated from the geometric centre ofthe beam passage 234 and/or the geometric centre of the photocathode243. In general the axis 245 is the axis with which it is desirable thatthe electron beam E is substantially coincident after the electron beamhas passed through the steering unit 240.

Electrons in an electron bunch emitted from the photocathode 243 areeach repelled away from each other by repulsive electrostatic forceswhich act between the electrons. This is the space charge effect and maycause the electron bunch to spread out. The spread of an electron bunchin position and momentum phase space may be characterised by theemittance of the electron beam E. Spreading out of electron bunches dueto the space charge effect increases the emittance of the electron beamE. It may be desirable for the electron beam E to have a low emittancein the linear accelerator 22, 122, 150 and the undulator 24, 124 (seeFIGS. 3 and 10) since this may increase the efficiency with which energyfrom the electrons is converted to radiation in the undulator 24.

In order to limit an increase in the emittance of the electron beam Ethe electron beam is accelerated in the electron booster 233.Accelerating an electron bunch in the electron booster 233 reduces thespread of the electron bunch caused by the space charge effect. It isadvantageous to accelerate the electron beam E (using the electronbooster 233) close to the photocathode 243 before the emittance of theelectron beam E increases substantially due to the space charge effect.

The electron booster 233 may, for example, accelerate electron bunchesto energies in excess of approximately 0.5 MeV. In some embodiments theelectron booster 233 may accelerate electron bunches to energies inexcess of approximately 5 MeV. In some embodiments the electron booster233 may accelerate electron bunches to energies of up to approximately10 MeV. The electron booster 233 may for example accelerate electronbunches to an energy of around 10 MeV.

In some embodiments the electron booster 233 may be positioneddownstream of the steering unit 240 as opposed to upstream of thesteering unit 240 as depicted in FIG. 12.

The electron booster 233 operate in the same manner the linearaccelerator 22 described above and may, for example, comprise aplurality of radio frequency cavities 247 (depicted in FIG. 12) and oneor more radio frequency power sources (not shown). The radio frequencypower sources may be operable to control electromagnetic fields alongthe axis 245 of the beam passage 234. As bunches of electrons passbetween the cavities 247, the electromagnetic fields controlled by theradio frequency power sources cause each bunch of electrons toaccelerate. The cavities 247 may be superconducting radio frequencycavities. Alternatively, the cavities 247 may be conventionallyconducting (i.e. not superconducting), and may be formed from, forexample, copper. The electron booster 233 may comprise a linearaccelerator.

In an alternative embodiment the electron booster 233 may, for example,comprise a laser accelerator, wherein the electron beam E passes througha focused laser beam and the electric field of the laser beam causes theelectrons to accelerate. Other types of electron boosters may also beused.

The electron beam E travels along the beam passage 234, and passeseither to a merging unit 33, 133 (when a LINAC is used) or directly to alinear accelerator (when a non-energy recovering linear accelerator isused). The beam passage 234 is pumped to vacuum pressure conditions butmay contain some residual gas molecules. The electron beam E may collidewith residual gas molecules and may ionize the gas molecules, therebycreating positively charged ions. The energy of the electrons increaseswhen they are accelerated, and this increased energy result in more ionsbeing created.

Positively charged ions throughout the free electron laser FEL areattracted to the path of the electron beam E whose negative charge actsas a potential well to the positively charged ions. The ions have asubstantially higher mass than electrons and as a result are notaccelerated for example by cavities 247 of the electron booster 233. Theions will diffuse along the beam passage 234 and may, for example,travel back into the injector 230. Ions reaching the injector 230 areattracted to the photocathode 243 due to the voltage of the photocathode243 and may collide with the photocathode 243.

The positive ions travelling back to the injector 230 will travel alongthe path of the electron beam E. The last portion of the path travelledby the ions towards the injector will be linear (e.g. corresponding withthe path between the injector 130 and the quadrupole magnet 183 shown inFIG. 11). This linear path may for example pass through a wall of a roomcontaining the injector 130. The linear path may be located within asolenoid. The linear path travelled by the positive ions may correspondwith the axis 245 shown in in FIG. 12.

Ion collisions with the photocathode 243 may damage the photocathode243. In particular, the collision of ions with the photocathode 243 maycause sputtering of material from the photocathode 243. Damage to thephotocathode 243 may cause a change in composition of the photocathode243 which may reduce the quantum efficiency of the photocathode 243 andtherefore reduce the peak current of the electron beam E which isemitted from the photocathode 243. Additionally or alternatively damageof the photocathode 243 by ion collisions may cause an increase in thesurface roughness of the photocathode 243. An increase in the surfaceroughness of the photocathode 243 may lead to an increase in theemittance of the electron beam E which is emitted from the photocathode243 and/or may lead to a reduction in the quantum efficiency of thephotocathode 243. Ion collisions with the photocathode 243 maytherefore, over time, reduce the peak current of the electron beam Eand/or increase the emittance of the electron beam E.

In addition to the effects of ion collisions with the photocathode 243,the laser beam 241, may cause damage to the region of the photocathode243 which is illuminated by the laser beam 241. Similarly to the effectsof ion collisions, the laser beam 241 may cause an increase in surfaceroughness and/or a change in composition of the photocathode 243 whichmay decrease the quantum efficiency of a region of the photocathode 243which is illuminated by the laser beam 241 and may increase theemittance of an electron beam E emitted from that region.

Damage to the photocathode 243 caused by ion collisions and/or the laserbeam 241, may reduce the useful lifetime of a photocathode. It maytherefore be desirable to reduce the damage to the photocathode 243and/or to reduce the impact of the damage to the photocathode 243 on thepeak current and the emittance of the electron beam E emitted from thephotocathode 243. This may increase the useful lifetime of thephotocathode 243.

FIG. 13 is a schematic depiction of the photocathode 243 as viewed alongthe axis 245. As mentioned above, the ions become aligned with theelectron beam E due to the potential well caused by the electron beam E.The path of the electron beam E may generally be substantiallycoincident with the axis 245 during passage from the injector 230. Inthe event that these ions enter the injector 230 and collide with thephotocathode 243 they will therefore collide with the photocathode 243close to the position at which the axis 245 meets the photocathode 243.The majority of the ions which collide with the photocathode 243 maytherefore impact the photocathode 243 in an impact region 249 (shown inFIG. 13) surrounding the axis 245 (which may correspond with thegeometric centre of the photocathode).

Ion collisions in the impact region 249 may alter the composition of thephotocathode 243 and/or increase the surface roughness of thephotocathode 243 in this region 249. An electron beam E which is emittedfrom the impact region 249 may therefore have a lower peak current (dueto a reduced quantum efficiency in the impact region 249) and/or ahigher emittance than an electron beam E which is emitted from a regionof the photocathode 243 other than the impact region 249. In order toincrease the peak current and/or to reduce the emittance of the electronbeam E, the laser beam 241 may be directed to be incident on anilluminated region 251 of the photocathode 243 which is separated fromthe axis 245 and from the impact region 249. For example, theilluminated region 251 may be separated from the axis 245 by a distanceof approximately a few millimeters. Relatively few ions collide with theilluminated region 251 since it is separated from the axis 245. Thecomposition and surface roughness of the illuminated region 251 may nottherefore be substantially reduced by ion collisions with thephotocathode 243 and thus an electron beam E with a high peak currentand low emittance may be emitted from the illuminated region 251.

Directing the laser beam 241 to be incident on an illuminated region 251of the photocathode 243 which is separate from the axis 245 causes theposition of the electron beam E which is emitted from the illuminatedregion 251 to be shifted from the axis 245 by a positional offset 253.When the electron beam E is emitted from the photocathode 243 at aposition separated from the axis 245, the electric field associated withthe photocathode 243 may cause the electron beam E to be emitted at anangle 252 which is not perpendicular with the surface of thephotocathode as is shown in FIG. 12. For example in embodiments in whichthe geometric centre of the photocathode 243 coincides with the axis245, only electrons which are emitted substantially at the axis 245 areemitted in a direction which is perpendicular to the surface of thephotocathode 243. The electron beam E may therefore be emitted with apositional displacement 253 and an angular displacement 252 from theaxis 245.

Subsequent components of the injector arrangement (e.g. a merging unitor a linear accelerator) may be configured to accelerate an electronbeam E whose position and trajectory is substantially coincident withthe axis 245. It may therefore be desirable to alter the trajectory ofthe electron beam E in order to correct for the positional displacement253 and the angular displacement 252 from the axis 245 such that theelectron beam E is substantially coincident with the axis 245 uponleaving the injector 230.

In order to align the electron beam E with the axis 245 the electronbeam E is adjusted with a steering unit 240 (shown in FIG. 12). Thesteering unit 240 is configured to alter the trajectory of the electronbeam E such that the electron beam E trajectory is substantiallycoincident with the axis 245 upon leaving the steering unit 240. Thesteering unit 240 may, for example, comprise one or more electromagnetsconfigured to generate a magnetic field in the beam passage 234. Themagnetic field may exert a force on the electron beam E which acts toalter the trajectory of the electron beam E. In the embodiment depictedin FIG. 12 the trajectory of the electron beam is altered by thesteering unit 240 until the electrons become substantially coincidentwith the axis 245.

In embodiments in which the steering unit comprises one or moreelectromagnets, the electromagnets may be arranged to form one or moreof a magnetic dipole, a magnetic quadrupole, a magnetic sextupole and/orany other kind of multipole magnetic field arrangement configured toapply a force to the electron beam E. The steering unit 240 mayadditionally or alternatively comprise one or more electrically chargedplates, configured to create an electric field in the beam passage 234such that a force is applied to the electron beam E. In general thesteering unit 240 may comprise any apparatus which is operable to applya force to the electron beam E to alter its trajectory so that theelectrons are coincident with the axis 245.

The mass to charge ratio of any ions which pass through the steeringunit 240 is much larger than the mass to charge ratio of electrons inthe electron beam E. The steering unit 240 does not thereforesubstantially adjust the position or direction of travel of ions whichpass through the steering unit 240 towards the photocathode 243. Ionspassing into the injector 230 (e.g. from the linear accelerator 22) mayhave enough momentum that the potential well created by the electronbeam E in the injector 230 does not substantially alter the path of theions in the injector 230. This has the effect of separating the path ofions (which may be substantially coincident with the axis 245) from theelectron beam E in the injector 230 and allows the position of theimpact region 249 on the photocathode 243 to be separated from theregion 251 of the photocathode 243 which is illuminated by the laserbeam 241. This ensures that the illuminated region 251 of thephotocathode 243 from which the electron beam E is emitted is separatedfrom the impact region 249 which is prone to damage from ion collisions.This may ensure that ion collisions with the photocathode 243 do notsubstantially reduce the peak current or increase the emittance of theelectron beam E.

However as was mentioned above the laser beam 241 which is incident onthe photocathode 243 may damage the region of the photocathode 243 whichis illuminated by the laser beam 241 over time and may reduce the peakcurrent and/or increase the emittance of the electron beam E which isemitted from the photocathode 243. In addition to damage of the region251 of the photocathode 243 which is illuminated by the laser beam 241which is caused by the laser beam 241, the illuminated region 251 mayalso be damaged by ions which are created in the injector 230 bycollisions between gas molecules and electrons which have not yet passedthrough the steering unit 240. Ions created before the steering unit 240may be attracted to the path of the electron beam E before the steeringunit 240 and may therefore diffuse along the path of the electron beam Eto collide with the region 251 of the photocathode 243 which isilluminated by the laser beam 241. However, in general fewer ions arecreated in the injector 230 than at positions downstream of the injectorand therefore ion damage to the region 251 of the photocathode 243 whichis illuminated by the laser beam 241 may be less significant than iondamage to the impact region 249.

In order to increase the useful lifetime of the photocathode 243, theregion 251 of the photocathode 243 which is illuminated by the laserbeam 241 may be changed. For example, after a period of time duringwhich the laser beam is incident on the illuminated region 251, duringwhich the illuminated region 251 may have become damaged, the positionof the laser beam 241 on the photocathode may be changed to a newilluminated region 251′ (as shown in FIG. 13). The laser beam 241 may beincident on the new illuminated region 251′ for a further period of timeuntil the new illuminated region 251′ becomes damaged. The region of thephotocathode 243 which is illuminated by the laser beam 241 may thenagain be changed to a further new illuminated region (not shown). Theposition of the region of the photocathode 243 which is illuminated bythe laser beam 241 may be recurrently changed, thereby scanning thelaser beam 241 over the photocathode 243, in order to illuminate newregions of the photocathode 243 which have not previously beenilluminated and therefore damaged.

This may allow a large region of the photocathode 243 to be used to emitthe electron beam E over the lifetime of the photocathode 243 and maytherefore increase the total useful lifetime of the photocathode 243.Changing the region of the photocathode 243 which is illuminated by thelaser beam 241 may, for example, allow the useful lifetime of thephotocathode 243 to be increased by a factor of 10 or more.

The region of the photocathode 243 which is illuminated by the laserbeam 241 may be changed continuously or may be changed in steps. In anembodiment in which the region of the photocathode 243 which isilluminated by the laser beam 241 is changed in steps, the steps mayoccur periodically. In an alternative embodiment the steps may occurnon-periodically.

The region of the photocathode 243 which is illuminated by the laserbeam 241 may, for example, be changed by a laser beam adjustment unit238 (shown in FIG. 12). The laser beam adjustment unit 238 may compriseone or more mirrors, lenses or other optical components suitable forchanging one or more properties of the laser beam 241. For example thelaser beam adjustment unit 238 may change the direction of propagationof the laser beam 241 such that the position at which the laser beam 241is incident on the mirror 239 and the position at which the laser beam241 is incident on the photocathode 243 is changed.

In an alternative embodiment the region of the photocathode 243 which isilluminated by the laser beam 241 may be changed by altering theposition and/or the orientation of the mirror 239. For example, themirror 239 may be tilted and/or moved in order to alter the direction inwhich the laser beam 241 is reflected from the mirror 239, therebychanging the region of the photocathode 243 which is illuminated by thelaser beam 241. The position and/or the orientation of the mirror 239may be changed using an actuator (not shown) which is operable to changethe position and/or the orientation of the mirror 239.

In a further alternative embodiment the region of the photocathode 243which is illuminated by the laser beam 241 may be changed by alteringthe position and/or the orientation of the laser 235. For example thelaser 235 may be tilted and/or moved in order to change the region ofthe photocathode 243 which is illuminated by the laser beam 241. Theposition and/or the orientation of the laser 235 may be changed by anactuator (not shown) which is operable to change the position and/or theorientation of the laser 235.

In a still further embodiment the region of the photocathode 243 whichis illuminated by the laser beam 241 may be changed by altering theposition and/or the orientation of the photocathode 243. For example,the photocathode 243 may be rotated whilst the position of the laserbeam 241 remains constant such that the region of the photocathode 243which is illuminated by the laser beam 241 rotates on the photocathode243. Alternatively both the position and/or the orientation of thephotocathode 243 and the laser beam 241 may be altered in order tochange the region of the photocathode 243 which is illuminated by thelaser beam 241.

The position and/or the orientation of the photocathode 243 may bechanged by an actuator (not shown). For example, the support structure242 may comprise an actuator which is operable to change the positionand/or the orientation of the photocathode 243.

A laser beam adjustment unit 238, an actuator operable to change theposition and/or the orientation of the mirror 239, an actuator operableto change the position and/or the orientation of the laser 235 and anactuator operable to change the position and/or the orientation of thephotocathode 243 are all examples of adjustment mechanisms operable tochange the region of the photocathode 243 which is illuminated by thelaser beam 241. It will be appreciated that other adjustment mechanismsmay be used to change the region of the photocathode 243 which isilluminated by the laser beam 241 without departing from the scope ofthe invention. Each adjustment mechanism may be used separately or incombination with one or more other adjustment mechanisms for changingthe region of the photocathode 243 which is illuminated by the laserbeam 241.

The region of the photocathode 243 which is illuminated by the laserbeam 241 may determine the angular displacement 252 and the positionaldisplacement 253 of the electron beam E from the axis 245. A change ofthe region of the photocathode 243 which is illuminated by the laserbeam 241 may therefore cause a corresponding change in the angularand/or positional displacement from the axis 245. In response to achange of the region of the photocathode 243 which is illuminated by thelaser beam 241, the steering unit 240 may adjust the force which isapplied to the beam of electrons E such that the trajectory of theelectron beam E is altered from its adjusted angular and positionaldisplacement so that the electron beam E continues to be coincident withthe axis 245 after the adjustment. The steering unit 240 may thereforebe operable to adjust the force which is applied to the beam ofelectrons E in response to the region of the photocathode 243 which isilluminated by the laser beam 241.

In an embodiment in which the steering unit 240 comprises one or moreelectromagnets the steering unit 240 may adjust one or more electricalcurrents which flow through coils of the one or more electromagnets. Anadjustment to the one or more electrical currents may cause a change inthe magnetic field which is generated by the steering unit 240 andtherefore change the force which is applied to the electron beam E bythe steering unit 240.

In an alternative embodiment the steering unit 240 may be mechanicallymoveable. For example, the steering unit 240 may be tilted, rotatedand/or shifted in order to adjust the force which is applied to theelectron beam E in response to a change of the region of thephotocathode 243 which is illuminated by the laser beam 241.

The steering unit 240 may be controlled by a controller 236 (shown inFIG. 12). The controller 236 may additionally or alternatively controlthe laser beam adjustment unit 238. The controller 236 may, for example,be a programmable logic controller. The controller 236 may cause thelaser beam adjustment unit 238 to change the region of the photocathode243 which is illuminated by the laser beam 241. The controller 236 mayfurther cause the steering unit 240 to adjust the force which is appliedto the electron beam E, in response to the change of the region of thephotocathode 243 which is illuminated by the laser beam 241, such thatthe electron beam E continues to be coincident with the axis 245 afterthe adjustment. The steering unit 240 may, for example, adjust the forcewhich is applied to the beam of electrons E by adjusting one or moreelectrical currents which flow through one or more electromagnets of thesteering unit 240.

In an alternative embodiment the controller 236 may control one or moreof the laser 235, the mirror 239 and the photocathode 243 in order tocause a change of the region of the photocathode 243 which isilluminated by the laser beam 241. In general the controller 236 maycontrol any adjustment mechanism which is operable to change the regionof the photocathode 243 which is illuminated by the laser beam 241.

In some embodiments the steering unit 240 may apply a force to the beamof electrons in response to a measurement of the electron beam E. Forexample, an electron beam measurement device (not shown) may bepositioned in proximity to the electron beam E and may measure theposition of the electron beam E. The steering unit 240 may use themeasurement of the electron beam E to compute and apply a force to theelectron beam E so that the electrons are coincident with the axis 245.When a change is made to the region of the photocathode on which thelaser beam 241 is incident, this may cause a change in position of theelectron beam E which may be measured by the electron beam measurementdevice. The electron beam measurement device may communicate the changein position of the electron beam to the steering unit 240. The steeringunit 240 may adjust the force which is applied to the electron beam E inresponse to the change in position of the electron beam E such that theforce applied by the steering unit alters the trajectory of the electronbeam E so that the electrons are coincident with the axis 245.

In an embodiment the laser beam measuring device and the controller 236may be used in combination. In such an embodiment, the laser beammeasuring device may be in communication with the controller 236.

In some embodiments the steering unit 240 may not adjust the force whichis applied to the electron beam E in response to a change of the regionof the photocathode 243 which is illuminated by the laser beam 241. Forexample in an embodiment in which the region of the photocathode 243which is illuminated by the laser beam 241 is changed by rotating thephotocathode 243 whilst the position of the laser beam 241 remainsconstant, the positional displacement 253 and the angular displacement252 of the electron beam E from the axis 245 do not change. In such anembodiment the trajectory of the electron beam E may continue to bealtered by the steering unit 240 such that the electrons are coincidentwith the axis 245 without an adjustment of the force applied by thesteering unit 240 on the electron beam E.

In some embodiments the position and/or the orientation of the electrongun 231 may be adjusted in response to a change in the region of thephotocathode 243 which is illuminated by the laser beam 241. For examplein response to a change in the region of the photocathode 243 which isilluminated by the laser beam 241 the position and/or the orientation ofthe electron gun 231 may be adjusted such that the trajectory of theelectrons leaving the electron gun 231 is substantially the same as thetrajectory of the electrons before the change in the region of thephotocathode 243 which is illuminated by the laser beam 241. In such anembodiment the force which is applied to the electron beam E by thesteering unit 240 may remain substantially constant. Alternatively boththe position and/or the orientation of the electron gun 231 and theforce which is applied to the electron beam E by the steering unit 240may be adjusted in response to a change in the region of thephotocathode 243 which is illuminated by the laser beam 241.

The position and/or the orientation of the electron gun 231 may beadjusted using an actuator (not shown) which is operable to change theposition and/or the orientation of the electron gun 231. In someembodiments the position and/or the orientation of the electron booster233 may also be adjusted along with an adjustment in the position and/orthe orientation of the electron gun 231 (for example using an actuator).

In some embodiments the region of the photocathode 243 which isilluminated by the laser beam 241 may be changed so that the laser beam241 is scanned across the photocathode 243. The laser beam 241 may bescanned over substantially all of the photocathode 243. Alternativelythe laser beam 241 may be scanned over substantially all of thephotocathode 243 except the impact region 249.

In other embodiments the laser beam 241 may only be scanned over aportion of the photocathode 243. For example, the region of thephotocathode 243 which is illuminated by the laser beam 241 may remaininside an inner portion 255 of the photocathode 243 (shown in FIG. 13).The inner portion 255 may correspond to the portion of the photocathode243 which lies within a distance R from the axis 245. The distance R mayrepresent the maximum positional displacement 253 of the electron beam Efrom the axis 245 for which the steering unit 240 may be operable toapply a force which adjusts the trajectory of the electron beam E sothat the electrons are coincident with the axis 245. For example anelectron beam E emitted from a region outside of the inner region 255may be so far displaced from the axis 245 that the electron beam E maycollide with the outer extent of the beam passage 234 before reachingthe steering unit 240. Alternatively the electron beam E may reach thesteering unit 240 at a displacement from the axis 245 which is too greatfor the steering unit 240 to alter the trajectory of the electron beam Eso that the electrons are coincident with the axis 245.

It may be desirable for electron bunches which leave the injector 230 tohave a particular shape and charge distribution. For example, it may bedesirable for an electron bunch to have a circular cross-section andhave an even charge density along its length. However the force which isapplied to the electron beam E by the steering unit 240, in addition toaltering the trajectory of the electron beam E, may also cause a changein the shape and/or the charge distribution of bunches of the electronbeam E. For example the steering unit 240 may compress or expand a bunchof electrons E in a particular direction.

In order to reduce the effects of any changes in the shape and/or chargedistribution of an electron bunch caused by the steering unit 240, theshape of the region of the photocathode 243 which is illuminated by thelaser beam 241 may be controlled to produce an electron bunch which hasa desirable shape and/or charge distribution after passing through thesteering unit 240. For example, the region of the photocathode 243 whichis illuminated by the laser beam 241 may be elliptically shaped so as tocause an electron bunch having an elliptical cross-section to be emittedfrom the photocathode 243. The electron bunch may subsequently becompressed by the steering unit 240 along a semi-major axis of theelliptical cross-section of the electron bunch such that the electronbunch is compressed into having a circular cross-section when it leavesthe injector 230.

In general the shape of the region of the photocathode 243 which isilluminated by the laser beam 241 may be controlled such that the beamof electrons emitted from the illuminated region takes on one or moredesired properties after the steering unit 240 applies a force to thebeam of electrons. The one or more desired properties may, for example,be a particular shape and/or charge distribution of electron bunches ofthe beam of electrons.

The force which is applied to the electron beam E by the steering unit240 may be different for different regions of the photocathode 243 whichare illuminated by the laser beam 241. Any change in the shape and/orcharge distribution of electron bunches caused by the steering unit 240may therefore be different for electron bunches which are emitted fromdifferent regions of the photocathode 243. For example, if the region ofthe photocathode 243 which is illuminated by the laser beam 241 is movedaway from the axis 245 then the steering unit 240 may increase the forcewhich is applied to the electrons E. This may also increase a change inthe shape and/or charge distribution of electron bunches in the electronbeam E caused by the steering unit 240.

In order to adapt to different changes in the shape and/or chargedistribution of electron bunches which are emitted from differentregions of the photocathode 243, the shape of the region of thephotocathode 243 which is illuminated by the laser beam 241 may beadjusted for different positions of the laser beam 241 on thephotocathode 243. For example if the region of the photocathode 243which is illuminated by the laser beam 241 is moved away from the axis245 then the eccentricity of an elliptically shaped illuminated regionof the photocathode 243 may be increased. This may increase theeccentricity of the cross-sectional shape of electron bunches emittedfrom the photocathode 243 in anticipation of a larger compression of theshape of the electron bunches in the steering unit 240. This may ensurethat the electron bunches which leave the steering unit 240 have adesirable shape and/or charge distribution.

The shape of the region of the photocathode 243 which is illuminated bythe laser beam 241 may be controlled by the laser beam adjustment unit238. For example the laser beam adjustment unit 238 may control theshape of the laser beam 241 such that the shape of region of thephotocathode on which the laser beam 241 is incident is controlled tocause emission of electron bunches having desired properties (e.g. adesired shape and/or charge distribution). The desired properties ofelectron bunches emitted from the photocathode 243 may take into accountany change in the properties of electron bunches which is expected tooccur in the steering unit 240. The desired properties of electronbunches emitted from the photocathode 243 may be different for differentregions of the photocathode 243 from which the electron bunches areemitted.

Additionally or alternatively the shape of the region of thephotocathode 243 which is illuminated by the laser beam 241 may becontrolled by controlling an actuator which is operable to change theposition and/or the orientation of the mirror 239.

As noted further above, by separating the illumination region 251 fromthe impact region 249, the electron beam E is emitted from a region ofthe photocathode which receives fewer ion collisions and therefore isless prone to damage caused by ion collisions. This may extend theperiod of time for which an electron beam having a high peak current anda low emittance may be emitted from the photocathode 243 and maytherefore extend the useful lifetime of the photocathode 243.

However, regions of the photocathode 243 outside of the impact region249 may still be affected by ion collisions in the impact region 249since ion collisions may cause sputtering of material from thephotocathode 243 in the impact region 249. Some of the material which issputtered from the impact region 249 may return to the photocathode 243and may be deposited on regions of the photocathode 243 outside of theimpact region 249. The sputtered material may, for example, be depositedinside a deposition region 254. The deposition region 254 may, forexample, overlap with the illuminated regions 251, 251′, as shown inFIG. 13.

Sputtered material which is deposited on a deposition region 254 of thephotocathode 243 may alter the chemical composition of all or part ofthe deposition region 254 of the photocathode 243. This may cause thequantum efficiency of all or part of the deposition region 254 to bereduced. For example, the quantum efficiency of all or part of theilluminated region 251 may be reduced by deposited sputtered material.Unless the power of the laser beam 241 is increased in response to areduction in the quantum efficiency of all or part of the illuminatedregion 251, then the peak current of the electron beam E which isemitted from the illuminated region 251 will be reduced.

Sputtered material may be deposited unevenly over the photocathode 243.For example more sputtered material may be deposited on regions of thephotocathode 243 which are closer to the impact region 249 than may bedeposited on regions which are further away from the impact region 249.This may, for example, cause radial gradients in the quantum efficiencyof the photocathode 243.

Radial gradients in the quantum efficiency of the photocathode 243 maycause an uneven charge distribution of electron bunches emitted from thephotocathode 243. For example, the quantum efficiency of the illuminatedregion 251 may be greater in a portion of the illuminated region 251which is further away from the impact region 249 than in a portion ofthe illuminated region 251 which is closer to the impact region 249.When a pulse of the laser beam 241 is incident on the illuminated region251, more electrons are therefore emitted from the portion of theilluminated region 251 which has a higher quantum efficiency than areemitted from the portion that has a lower quantum efficiency. Anelectron bunch will therefore be emitted which has an uneven chargedistribution. This may be disadvantageous since the efficiency withwhich energy from electron bunches are converted to radiation in theundulator 24 may be reduced for electron bunches having an uneven chargedistribution.

An uneven distribution of quantum efficiency of a photocathode 243caused by sputtered material being deposited on the photocathode 243 mayalso cause instabilities in the current of an electron beam E which isemitted from the photocathode 243. The current instabilities may beerratic in nature and may have a high frequency. Such currentinstabilities are difficult to anticipate and correct for, for example,by modulating the power of the laser beam 241.

In addition to causing sputtering of material from the photocathode 243,ions which collide with the photocathode 243 may transfer some of theirenergy to the photocathode 243 in the form of heat energy. This maycause regions of the photocathode 243 to be heated. Heating of thephotocathode 243 may cause some thermionic emission of electrons fromthe photocathode 243. Thermionic emission of electrons from thephotocathode 243 may occur during pulses of the laser beam 241 and attimes between pulses of the laser beam 241. Thus electrons may beemitted at times during which the photocathode 243 is not illuminated bythe laser beam 241. Electrons which are emitted at times when thephotocathode 243 is not illuminated by the laser beam 241 causes a flowof electrons from the photocathode 243 which is referred to as a darkcurrent.

Dark current electrons are not synchronized with the high frequencyreversing electromagnetic fields in the electron booster 233 or thelinear accelerator(s) 22, 122, 150. Dark current electrons may thereforetake on a wide range of energies depending on the time at which theyarrive at the electron booster 233 and/or the linear accelerator(s) 22,122, 150. The wide range of energies of dark current electrons meansthat elements of the free electron laser FEL which may be designed todirect or focus electrons in the free electron laser FEL may havevarying effects on the dark current electrons. The dark currentelectrons may therefore be stray electrons which are difficult tocontrol.

The stray dark current electrons may collide with elements of the freeelectron laser FEL which may cause damage to elements of the freeelectron laser. For example, dark current electrons may collide withmagnetic components of the undulator 24, 124. This may causedemagnetization of the undulator which may reduce the useful lifetime ofthe undulator. Electrons may also collide with other components of thefree electron laser such as the outer extent of the beam passage 234which may cause the beam passage 234 to become radioactive.

As was described above, ion collisions with the photocathode 243 maycause sputtering of material from the photocathode 243 and/or heating ofthe photocathode 243 leading to an increased emission of a dark currentfrom the photocathode 243. Both of these effects may havedisadvantageous consequences in a free electron laser as was describedabove. It is therefore desirable to provide a photocathode 243 whichreduces or obviates sputtering of material from the photocathode 243and/or reduces heating of the photocathode 243 in the event of ioncollisions with the photocathode 243.

FIG. 14 is a schematic depiction of a photocathode according to anembodiment of the invention. FIG. 14 is a cross-section of thephotocathode 243 (which may have the form shown in FIG. 13 when viewedfrom above). The photocathode 243 comprises a substrate 261 on which afilm of material 263 is disposed. The substrate may, for example,comprise silicon, molybdenum, stainless steel or another suitablematerial. The substrate 261 may be polished in order to betterfacilitate deposition of the film of material 263 on the substrate 261.

The film of material 263 may comprise a material having a high quantumefficiency. The film of material 263 may, for example, have a quantumefficiency of a few percent. For example, the film of material 263 mayhave a quantum efficiency of approximately 5% (which may be consideredto be a high quantum efficiency). The film of material 263 has a surface264 on which the laser beam 241 is incident and from which a beam ofelectrons E is emitted. The surface 264 may be referred to as anelectron emitting surface.

The film of material 263 may be a material comprising one or more alkalimetals. The film of material 263 may be a compound comprising one ormore alkali metals and antimony. For example, the film of material 263may comprise sodium potassium antimonide. Components of the film ofmaterial 263 may be individually deposited on to the substrate 261. Forexample, in an embodiment in which the film of material 263 comprisessodium potassium antimonide, the antimonide may first be deposited on tothe substrate followed by the potassium and then the sodium. The film ofmaterial 263 may be deposited onto the substrate 261 by atomic vapourdeposition. The substrate 261 may be heated during the depositionprocess.

The thickness 267 of the film of material 263 may be significantly lessthan 1 micron. For example, the thickness 267 of the film of material263 may be approximately a few tens of nanometers. The thickness 269 ofthe substrate 261 may be approximately a few millimetres. For example,the thickness 269 of the substrate 261 may be between 1 and 10millimetres.

A photocathode 243 may alternatively be formed from a substrate 261comprising a material which is configured to emit electrons whenilluminated by the laser beam 241. Such a photocathode 243 may not havea film of material 263 disposed on the substrate 261. The electronemitting surface 264 may instead be a surface of the substrate 261. Forexample, the photocathode 243 may comprise a copper substrate 261 whichincludes an electron emitting surface. However, copper may have arelatively low quantum efficiency compared to, for example, a film ofmaterial comprising one or more alkali metals. Such a photocathode maynot therefore be suitable for use in a free electron laser since aphotocathode comprising a material having a high quantum efficiency isadvantageous in a free electron laser.

In order to reduce the effects of ion collisions with the photocathode243, a cavity 265 is formed in the substrate 261. The cavity 265 may besubstantially aligned with the impact region 249. For example the cavity265 may be positioned substantially beneath the impact region 249 asshown in FIG. 14. The cavity 265 may radially extend beyond the extentof the impact region 249 as is shown in FIG. 14. However, the impactregion 249 is not a well-defined region but merely represents a regionof the photocathode 243 with which ions may collide. In general thecavity 265 may be substantially aligned with a region of thephotocathode 243 with which ions collide such that the ions may passinto the cavity 265. The cavity may be substantially aligned with anaxis 245 of the injector 230 (see FIG. 12). The axis of the injector maycorrespond with a desired path of the electron beam E on leaving theinjector 230.

The cavity 265 may be separated from the film of material 263 by a thinlayer of the substrate 261 having a thickness 266. This may allow thecavity 265 to be positioned in close proximity to the film of material263 whilst leaving a layer of substrate 261 having a smooth surface onwhich the film of material 263 may be deposited. The thin layer of thesubstrate 261 which separates the film of material 263 and the cavity265 may, for example, have a thickness 266 which is betweenapproximately 0.1 micron and approximately 10 microns.

Ions which collide with a photocathode pass through an upper layer ofthe photocathode before being stopped at a particular depth into thephotocathode. The ions do not significantly interact with the portion ofthe photocathode through which they pass before being stopped by thephotocathode. The majority of the damage caused by colliding ionstherefore occurs at a depth into the photocathode which is at or closeto the depth at which the ions are stopped by the photocathode.

Ions which collide with the photocathode 243 may therefore travelthrough the film of material 263, through the thin layer of thesubstrate 261 and into the cavity 265 without being stopped. The film ofmaterial 263 and the portion of the substrate 261 through which the ionspass may act to partially decelerate the ions but this deceleration maynot be sufficient to stop the ions. Once the ions are in the cavity 265there is no substrate material to decelerate them and they may passthrough the cavity 265 without being further substantially decelerated.The ions may therefore pass out of the opposite side (i.e. opposite fromthe side through which they entered) of the cavity 265 and intosubstrate material below the cavity 265. The substrate material mayserve to further decelerate the ions and stop the ions. The point atwhich the ions are stopped may therefore be below the cavity 265 becausethe ions pass through the cavity 265 before being decelerated andstopped by substrate material below the cavity 265. The cavity 265therefore has the effect of shifting the position at which ions arestopped in the photocathode 261 to a greater depth into the photocathode243 and away from the electron emitting surface 264 of the photocathode243.

In an alternative embodiment the cavity 265 may extend through the rearof the substrate 261 such that the cavity 265 is open to thesurroundings or another material. In such an embodiment the ions maypass through the cavity 265 and out of the photocathode 243 such thatany damage caused by the ions occurs outside of the photocathode 243.

Increasing the depth at which ions are stopped in the photocathode 243may cause less sputtering of material at the electron emitting surface264 of the photocathode 243. Increasing the depth at which ions arestopped in the photocathode 243 may increase the depth at which energyis transferred from the ions to the photocathode 243 in the form of heatenergy. This reduces the heating of the photocathode 243 close to theelectron emitting surface 264 of the photocathode 243 and thereforereduces thermionic emission of electrons from the electron emittingsurface 264 of the photocathode 243, thereby reducing the dark currentin the free electron laser FEL. The cavity 265 may additionally reducethe amount of ions which may be retained in the substrate 261 near tothe surface 264 and may therefore reduce any blistering of the surface264 which may otherwise occur.

The principles behind the invention may be further understood withreference to FIG. 15. FIG. 15 is a representation of the results of asimulation of positive hydrogen ions having energies of 500 keVcolliding with a silicon substrate. An energy of 500 keV mayapproximately correspond to the energy of ions in a free electron laser.The dots represent positions in the substrate at which the hydrogen ionswere stopped by the silicon substrate. As can be seen in FIG. 15 themajority of ions are stopped at a depth into the silicon target which isgreater than approximately 4 microns. As was mentioned above the film ofmaterial 263 may have a thickness 267 which is much less than 1 micron.The majority of ions will therefore pass through the film of material263 without being stopped.

The results displayed in FIG. 15 indicate that in an embodiment in whichthe substrate 261 comprises silicon and the thin layer of the substrate261 which separates the cavity 265 from the layer of material 263 has athickness 266 which is less than approximately 4 microns, then themajority of ions will pass through the thin layer of the substrate andinto the cavity 265. Ions which pass into the cavity 265 will pass outof another side of the cavity 265 without being substantiallydecelerated. The ions will not therefore be stopped until passing intosubstrate material beyond the cavity.

It will be appreciated that different materials of a substrate 261 andions having different energies may be stopped at different positions ina substrate than is shown in FIG. 15. The dimensions of a photocathode243 and the arrangement of a cavity 265 may therefore be selecteddepending on the materials of the photocathode 243 and on the intendeduse of the photocathode 243.

In general, the cavity 265 in the substrate 261 may be positioned suchthat the thickness of a portion of the photocathode 243 disposed betweenthe surface 264 and the cavity 265 is sufficiently thin that themajority of ions incident at that portion of the photocathode 243 passthrough that portion of the photocathode 243 and into the cavity 265.The thickness of the portion of the photocathode disposed between thesurface 264 and the cavity 265 may for example be less thanapproximately 10 microns, and may be less than approximately 5 microns.

During operation, a photocathode 243 may be subjected to electrostaticpressure resulting from the electric field associated with the voltageof the photocathode 243. The electric field associated with the voltageof the photocathode 243 may, for example, have a field strength ofapproximately 10 MVm⁻¹. This may cause the photocathode 243 to besubjected to an electrostatic pressure of approximately 1000 Pascals.

FIG. 16 is a schematic depiction of the same cross-section of aphotocathode 243 as shown in FIG. 14 when electrostatic pressure isapplied to the photocathode. The direction of the electrostatic pressureon the photocathode 243 is indicated by arrows 272. The cavity 265 maystructurally weaken the photocathode 243 in the region of the cavity265. For example, the thin layer of the photocathode 243 which liesimmediately above the cavity 265 may not be strong enough to withholdthe electrostatic pressure to which it is subjected. This may cause aregion 273 of the photocathode 243 to be deformed under theelectrostatic pressure.

Deformation of the photocathode 243 may cause an alteration of theelectric field in the deformed region 273. The direction of the electricfield around the photocathode 243 is indicated by arrows 271 in FIG. 16.The direction of the electric field in the deformed region 273 isaltered by the deformation to the photocathode 243. In particular theelectric field 271 may be focused in the deformed region 273. This maybe disadvantageous since it may alter the emittance and trajectory ofelectron bunches which are emitted from the photocathode 243. It maytherefore be advantageous to reduce any deformation of the photocathode243 caused by electrostatic pressure and/or to reduce or mitigate theeffect of any deformation of the photocathode 243 on the electric fieldaround the photocathode 243.

FIG. 17a is a schematic depiction of a photocathode 243 before a voltageis applied to the photocathode 243. Because no voltage is being appliedto the photocathode 243 the photocathode is not subjected to anyelectrostatic pressure. The photocathode 243, shown in FIG. 17a , isshaped to anticipate being deformed under electrostatic pressure once avoltage is applied to the photocathode 243. That is the photocathode 243is configured to take on a desired shape after being deformed underelectrostatic pressure. In particular the substrate 261 is shaped tocomprise an indentation 275 in the region above the cavity 265.

Optionally the cavity 265 may also be shaped in anticipation ofexperiencing electrostatic pressure. For example, the cavity 265 may beshaped to include a chamfer 276 as depicted in FIG. 17a . The chamfer276 acts to reduce stress in regions of the substrate 261 which surroundcorners of the cavity 265. This may reduce the risk of cracks developingin the substrate in these regions as a result of the electrostaticpressure applied to the photocathode 243.

FIG. 17b is a schematic depiction of the same photocathode 243 as wasdepicted in FIG. 17a when a voltage is applied to the photocathode 243.The electrostatic pressure 272 to which the photocathode 243 issubjected causes a deformation of the previously indented region 275 ofthe substrate 261. After deformation the previously indented region 275is no longer indented and the upper surface of the substrate 261 issubstantially flat. After deformation of the previously indented region275 the electric field direction 271 is substantially uniform and thuselectron bunches emitted from the photocathode 243 may be substantiallyunaffected by the presence of the cavity 265.

The photocathode 243 depicted in FIGS. 17a and 17b is purely an exampleof an embodiment of a photocathode 243. It will be appreciated thatother photocathodes 243 may be shaped and configured differently inanticipation of electrostatic pressure which may be exerted on thephotocathode 243. The particular shaping and configuration of aphotocathode 243 may depend amongst other things on the materials fromwhich the photocathode 243 is constructed and the intended operatingconditions of the photocathode 243.

Alternatively a photocathode 243 may be constructed in order to resistan electrostatic pressure such that the electrostatic pressure does notsubstantially deform the photocathode 243. For example the substrate 261and the film of material 263 in the region of the cavity 265 may besufficiently stiff to withstand an electrostatic pressure which isexpected to be applied to the photocathode 243 during operation.

In order to strengthen a photocathode 243, reinforcing ribs may added tothe substrate 261. The reinforcing ribs may help to resist deformationunder an electrostatic pressure. FIG. 18 is plan view of a portion ofsubstrate 261 comprising reinforcing ribs 277. The reinforcing ribs 277are arranged in a honeycomb structure and may act to strengthen thesubstrate 261. The reinforcing ribs 277 may additionally help to coolthe substrate 261. The ribs 277 may, for example, radiate heat from thesubstrate 261 to the surroundings thereby cooling the substrate 261. Inalternative embodiments the reinforcing ribs 277 may be arranged to forma structure other than a honeycomb structure.

It may be desirable that the reinforcing ribs 277 do not substantiallyprevent ions from passing through the substrate 261. For example, it maybe desirable that the reinforcing ribs do not substantially prevent ionsfrom passing into the cavity 265. This may be achieved by providing asubstantial separation between reinforcing ribs 277 (relative to thethickness of the ribs). This may ensure that the fraction of thesubstrate 261 which is occupied up by the reinforcing ribs 277 isrelatively small (e.g. less than 10%). The proportion of ions incidenton the substrate 261 which collide with the ribs 277 is thereforerelatively small and thus an insubstantial proportion of ions areprevented from passing through the substrate 261 by the reinforcing ribs277.

Additionally or alternatively the thickness of the ribs 277 may besufficiently thin that ions may pass through the ribs 277 such that theribs 277 do not substantially prevent ions from passing through thesubstrate 261. The thickness of the reinforcing ribs 277 may, forexample, be less than approximately 1 micron.

The reinforcing ribs 277 may be positioned not throughout the substrate261 but may in particular be positioned in portions of the substrate 261which are subjected to large amounts of stress when a voltage is appliedto the photocathode 243. For example, the reinforcing ribs 277 may bepositioned to reinforce the layer of the substrate 261 which separatesthe film of material 263 and the cavity 265 since this layer of thesubstrate 261 may be subjected to a large amount of stress during use.

Embodiments of a photocathode 243 have been described above ascomprising a cavity 265 formed in a substrate 261 which is enclosed bythe material of the substrate 261. However in some embodiments thecavity 265 may be open to the surroundings or other materials. FIG. 19is a schematic depiction of a photocathode 243 comprising a cavity 265which extends to the base of the substrate 261. Ions which pass into thecavity 265 may therefore travel through the cavity and out of an opening278 formed by the cavity 265. A material may optionally be positionedbeyond the opening 278 in order to absorb and stop ions which passthrough the opening. This may ensure that any damage caused by the ionsoccurs outside of the photocathode 243.

Although embodiments of an electron source have been described inrelation to a laser 235 which emits a laser beam 241 which is incidenton a photocathode 243, it should be appreciated that in otherembodiments a radiation source other than a laser may be used. Theradiation source may emit a radiation beam which is not a laser beam.Any reference above to a laser and/or a laser beam may therefore, insome embodiments, be substituted for a radiation source and/or aradiation beam respectively.

Although embodiments of a free electron laser have been described ascomprising a linear accelerator 22, 150, 122, it should be appreciatedthat a linear accelerator is merely an example of a type of particleaccelerator which may be used to accelerate electrons in a free electronlaser. A linear accelerator may be particularly advantageous since itallows electrons having different energies to be accelerated along thesame trajectory. However in alternative embodiments of a free electronlaser other types of particle accelerators may be used to accelerateelectrons to relativistic speeds.

The term “relativistic electrons” should be interpreted to meanelectrons which travel at relativistic speeds. In particularrelativistic electrons may be used to refer to electrons which have beenaccelerated to relativistic speeds by a particle accelerator.

It will be appreciated that the injector and injector arrangementsdepicted in the Figures are merely embodiments, and that other injectorarrangements in accordance with the present invention are possible.

It is to be understood that while a lithographic system comprising eightlithographic apparatus LA1-LA8 is referred to above, more or fewerlithographic apparatus may be provided in a lithographic system.Further, one or more of the lithographic apparatus LA1-LA8 may comprisea mask inspection apparatus MIA. In some embodiments, the lithographicsystem may comprise two mask inspection apparatuses to allow for someredundancy. This may allow one mask inspection apparatus to be used whenthe other mask inspection apparatus is being repaired or undergoingmaintenance. Thus, one mask inspection apparatus is always available foruse. A mask inspection apparatus may use a lower power radiation beamthan a lithographic apparatus.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

Embodiments of the invention may form part of a lithographic apparatuswhich uses an electron beam or multiple electron beams to pattern asubstrate.

The term “EUV radiation” may be considered to encompass electromagneticradiation having a wavelength within the range of 5-20 nm, for examplewithin the range of 13-14 nm. EUV radiation may have a wavelength ofless than 10 nm, for example within the range of 5-10 nm such as 6.7 nmor 6.8 nm.

The lithographic apparatuses LA1 to LA8 may be used in the manufactureof ICs. Alternatively, the lithographic apparatuses LA1 to LA8 describedherein may have other applications. Possible other applications includethe manufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, flat-panel displays,liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

The invention claimed is:
 1. A photocathode comprising: a substrate inwhich a cavity is formed; and a film of material disposed on thesubstrate, wherein the film of material comprises an electron emittingsurface configured to emit electrons when illuminated by a beam ofradiation at an impact region, wherein the impact region is configuredto receive ions during operation of the photocathode, wherein theelectron emitting surface is on an opposite side of the film of materialfrom the cavity, wherein the cavity is substantially aligned with theimpact region, and wherein the cavity is configured to reduce effects ofion collisions with the photocathode.
 2. The photocathode of claim 1,wherein the thickness of a portion of the photocathode disposed betweenthe electron emitting surface and the cavity is sufficiently thin thatthe majority of positively charged ions incident at that portion of thephotocathode pass through that portion of the photocathode and into thecavity.
 3. The photocathode of claim 2, wherein the thickness of theportion of the photocathode disposed between the electron emittingsurface and the cavity is less than 10 microns.
 4. The photocathode ofclaim 1, wherein the photocathode is configured to take on a desiredshape after a deformation of the photocathode which is brought about byan electrostatic pressure applied to the photocathode when thephotocathode is held at a voltage.
 5. The photocathode of claim 4,wherein the photocathode is configured such that after the deformationof the photocathode, electric field lines associated with the voltageapplied to the photocathode are substantially uniform.
 6. Thephotocathode of claim 4, wherein the substrate comprises an indentationin the substrate.
 7. The photocathode of claim 4, wherein the cavity inthe substrate comprises a chamfer.
 8. The photocathode of any of claim1, wherein the substrate comprises one or more ribs.
 9. The photocathodeof claim 8, wherein the one or more ribs are arranged to strengthen thephotocathode to resist an electrostatic pressure applied to thephotocathode when the photocathode is held at a voltage.
 10. Thephotocathode of claim 8, wherein the ribs are arranged in a honeycombstructure.
 11. The photocathode of claim 8, wherein the ribs have athickness of less than approximately 1 micron.
 12. The photocathode ofclaim 1, wherein the substrate comprises silicon.
 13. The photocathodeof claim 1, wherein the film of material comprises one or more alkalimetals.
 14. The photocathode of claim 13, wherein the film of materialcomprises sodium potassium antimonide.
 15. An electron injectorcomprising: a photocathode arranged to receive a beam of radiation froma radiation source, the photocathode comprising: a substrate in which acavity is formed; a film of material disposed on the substrate, whereinthe film of material comprises an electron emitting surface configuredto emit electrons when illuminated by a beam of radiation at an impactregion, wherein the impact region is configured to receive ions duringoperation of the photocathode, wherein the electron emitting surface ison an opposite side of the film of material from the cavity, wherein thecavity is substantially aligned with the impact region, and wherein thecavity is configured to reduce effects of ion collisions with thephotocathode, and an electron booster operable to accelerate a beam ofelectrons emitted from the photocathode.
 16. A free electron lasercomprising: an electron injector comprising; a photocathode arranged toreceive a beam of radiation from a radiation source, the photocathodecomprising: a substrate in which a cavity is formed; a film of materialdisposed on the substrate, wherein the film of material comprises anelectron emitting surface configured to emit electrons when illuminatedby a beam of radiation at an impact region, wherein the impact region isconfigured to receive ions during operation of the photocathode, whereinthe electron emitting surface is on an opposite side of the film ofmaterial from the cavity, wherein the cavity is substantially alignedwith the impact region, and wherein the cavity is configured to reduceeffects of ion collisions with the photocathode, and an electron boosteroperable to accelerate a beam of electrons emitted from thephotocathode, a linear accelerator operable to accelerate the beam ofelectrons to relativistic speeds; and an undulator operable to cause therelativistic electrons to follow an oscillating path thereby causingthem to stimulate emission of coherent radiation.
 17. The free electronlaser of claim 16, wherein the undulator is configured to cause theelectrons to emit EUV radiation.
 18. The free electron laser of claim16, wherein the linear accelerator is an energy recovery linearaccelerator, and wherein the free electron laser further comprises amerging unit configured to combine the electron beam output from theelectron injector with a recirculating electron beam.
 19. A lithographicsystem comprising: an illumination system configured to provide a beamof radiation; a support configured to support a patterning device thatis configured to impart a pattern on the beam of radiation; a projectionsystem configured to direct the patterned beam of radiation onto atarget portion of a substrate; a free electron laser comprising anelectron injector comprising a photocathode, arranged to receive a beamof radiation from a radiation source, the photocathode comprising: asubstrate in which a cavity is formed; a film of material disposed onthe substrate, wherein the film of material comprises an electronemitting surface configured to emit electrons when illuminated by a beamof radiation at an impact region, wherein the impact region isconfigured to receive ions during operation of the photocathode, whereinthe electron emitting surface is on an opposite side of the film ofmaterial from the cavity, wherein the cavity is substantially alignedwith the impact region, and wherein the cavity is configured to reduceeffects of ion collisions with the photocathode, an electron boosteroperable to accelerate a beam of electrons emitted from thephotocathode, a linear accelerator operable to accelerate the beam ofelectrons to relativistic speeds; and an undulator operable to cause therelativistic electrons to follow an oscillating path thereby causingthem to stimulate emission of coherent radiation.
 20. A method ofproducing an electron beam comprising: directing a beam of radiation tobe incident on an impact region of a photocathode, the impact regionreceiving ions during operation of the photocathode, thereby causing thephotocathode to emit a beam of electrons, the photocathode comprising asubstrate in which a cavity is formed, the cavity being substantiallyaligned with the impact region to reduce the effects of ion collisionswith the photocathode, and a film of material disposed on the substrate;and emitting electrons from an electron emitting surface of the filmwhen illuminated by the beam of radiation, the electron emitting surfacebeing on an opposite side of the film of material from the cavity.